20-F 1 nggtf-20210331.htm 20-F nggtf-20210331
00010043152021FYFalse12.43129595332567813574991697200010043152020-04-012021-03-310001004315dei:BusinessContactMember2020-04-012021-03-310001004315ifrs-full:OrdinarySharesMember2020-04-012021-03-310001004315nggtf:AmericanDepositarySharesMember2020-04-012021-03-310001004315nggtf:A390SeriesPreferredStockMember2020-04-012021-03-310001004315nggtf:A360SeriesPreferredStockMember2020-04-012021-03-31xbrli:shares00010043152021-03-31iso4217:GBPiso4217:GBPxbrli:shares00010043152019-04-012020-03-3100010043152018-04-012019-03-310001004315ifrs-full:DiscontinuedOperationsMember2020-04-012021-03-310001004315ifrs-full:DiscontinuedOperationsMember2019-04-012020-03-310001004315ifrs-full:DiscontinuedOperationsMember2018-04-012019-03-310001004315ifrs-full:AggregateContinuingAndDiscontinuedOperationsMember2020-04-012021-03-310001004315ifrs-full:AggregateContinuingAndDiscontinuedOperationsMember2019-04-012020-03-310001004315ifrs-full:AggregateContinuingAndDiscontinuedOperationsMember2018-04-012019-03-310001004315ifrs-full:IssuedCapitalMemberifrs-full:PreviouslyStatedMember2018-03-310001004315ifrs-full:SharePremiumMemberifrs-full:PreviouslyStatedMember2018-03-310001004315ifrs-full:RetainedEarningsMemberifrs-full:PreviouslyStatedMember2018-03-310001004315ifrs-full:PreviouslyStatedMembernggtf:OtherEquityReservesMember2018-03-310001004315ifrs-full:PreviouslyStatedMemberifrs-full:EquityAttributableToOwnersOfParentMember2018-03-310001004315ifrs-full:PreviouslyStatedMemberifrs-full:NoncontrollingInterestsMember2018-03-310001004315ifrs-full:PreviouslyStatedMember2018-03-310001004315ifrs-full:RetainedEarningsMemberifrs-full:FinancialEffectOfCorrectionsOfAccountingErrorsMember2018-03-310001004315ifrs-full:EquityAttributableToOwnersOfParentMemberifrs-full:FinancialEffectOfCorrectionsOfAccountingErrorsMember2018-03-310001004315ifrs-full:FinancialEffectOfCorrectionsOfAccountingErrorsMember2018-03-310001004315ifrs-full:RetainedEarningsMemberifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMember2018-04-010001004315ifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMembernggtf:OtherEquityReservesMember2018-04-010001004315ifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMemberifrs-full:EquityAttributableToOwnersOfParentMember2018-04-010001004315ifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMember2018-04-010001004315ifrs-full:IssuedCapitalMember2018-04-010001004315ifrs-full:SharePremiumMember2018-04-010001004315ifrs-full:RetainedEarningsMember2018-04-010001004315nggtf:OtherEquityReservesMember2018-04-010001004315ifrs-full:EquityAttributableToOwnersOfParentMember2018-04-010001004315ifrs-full:NoncontrollingInterestsMember2018-04-0100010043152018-04-010001004315ifrs-full:RetainedEarningsMember2018-04-012019-03-310001004315nggtf:OtherEquityReservesMember2018-04-012019-03-310001004315ifrs-full:EquityAttributableToOwnersOfParentMember2018-04-012019-03-310001004315ifrs-full:NoncontrollingInterestsMember2018-04-012019-03-310001004315ifrs-full:IssuedCapitalMember2018-04-012019-03-310001004315ifrs-full:SharePremiumMember2018-04-012019-03-310001004315ifrs-full:IssuedCapitalMember2019-03-310001004315ifrs-full:SharePremiumMember2019-03-310001004315ifrs-full:RetainedEarningsMember2019-03-310001004315nggtf:OtherEquityReservesMember2019-03-310001004315ifrs-full:EquityAttributableToOwnersOfParentMember2019-03-310001004315ifrs-full:NoncontrollingInterestsMember2019-03-3100010043152019-03-310001004315ifrs-full:RetainedEarningsMember2019-04-012020-03-310001004315ifrs-full:EquityAttributableToOwnersOfParentMember2019-04-012020-03-310001004315ifrs-full:NoncontrollingInterestsMember2019-04-012020-03-310001004315nggtf:OtherEquityReservesMember2019-04-012020-03-310001004315ifrs-full:IssuedCapitalMember2019-04-012020-03-310001004315ifrs-full:SharePremiumMember2019-04-012020-03-310001004315ifrs-full:IssuedCapitalMember2020-03-310001004315ifrs-full:SharePremiumMember2020-03-310001004315ifrs-full:RetainedEarningsMember2020-03-310001004315nggtf:OtherEquityReservesMember2020-03-310001004315ifrs-full:EquityAttributableToOwnersOfParentMember2020-03-310001004315ifrs-full:NoncontrollingInterestsMember2020-03-3100010043152020-03-310001004315ifrs-full:RetainedEarningsMember2020-04-012021-03-310001004315nggtf:OtherEquityReservesMember2020-04-012021-03-310001004315ifrs-full:EquityAttributableToOwnersOfParentMember2020-04-012021-03-310001004315ifrs-full:NoncontrollingInterestsMember2020-04-012021-03-310001004315ifrs-full:IssuedCapitalMember2020-04-012021-03-310001004315ifrs-full:SharePremiumMember2020-04-012021-03-310001004315ifrs-full:IssuedCapitalMember2021-03-310001004315ifrs-full:SharePremiumMember2021-03-310001004315ifrs-full:RetainedEarningsMember2021-03-310001004315nggtf:OtherEquityReservesMember2021-03-310001004315ifrs-full:EquityAttributableToOwnersOfParentMember2021-03-310001004315ifrs-full:NoncontrollingInterestsMember2021-03-3100010043152018-03-310001004315nggtf:FacilitiesToFinanceTheAcquisitionMember2021-03-310001004315nggtf:FacilitiesToBackupAcquiredDebtMember2021-03-310001004315nggtf:FixedInterestRateBondsMember2021-03-31xbrli:pure0001004315nggtf:NarragansettElectricCompanyMember2021-03-172021-03-170001004315ifrs-full:FinancialEffectOfCorrectionsOfAccountingErrorsMember2019-03-310001004315ifrs-full:FinancialEffectOfCorrectionsOfAccountingErrorsMember2020-03-31nggtf:segment0001004315ifrs-full:OperatingSegmentsMembernggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315ifrs-full:EliminationOfIntersegmentAmountsMembernggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315nggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315ifrs-full:EliminationOfIntersegmentAmountsMembernggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315nggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315ifrs-full:EliminationOfIntersegmentAmountsMembernggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315nggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMember2020-04-012021-03-310001004315nggtf:UKGasTransmissionMemberifrs-full:EliminationOfIntersegmentAmountsMember2020-04-012021-03-310001004315nggtf:UKGasTransmissionMember2020-04-012021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMember2019-04-012020-03-310001004315nggtf:UKGasTransmissionMemberifrs-full:EliminationOfIntersegmentAmountsMember2019-04-012020-03-310001004315nggtf:UKGasTransmissionMember2019-04-012020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMember2018-04-012019-03-310001004315nggtf:UKGasTransmissionMemberifrs-full:EliminationOfIntersegmentAmountsMember2018-04-012019-03-310001004315nggtf:UKGasTransmissionMember2018-04-012019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMember2020-04-012021-03-310001004315nggtf:USRegulatedMemberifrs-full:EliminationOfIntersegmentAmountsMember2020-04-012021-03-310001004315nggtf:USRegulatedMember2020-04-012021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMember2019-04-012020-03-310001004315nggtf:USRegulatedMemberifrs-full:EliminationOfIntersegmentAmountsMember2019-04-012020-03-310001004315nggtf:USRegulatedMember2019-04-012020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMember2018-04-012019-03-310001004315nggtf:USRegulatedMemberifrs-full:EliminationOfIntersegmentAmountsMember2018-04-012019-03-310001004315nggtf:USRegulatedMember2018-04-012019-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMember2020-04-012021-03-310001004315nggtf:NGVenturesAndOtherMemberifrs-full:EliminationOfIntersegmentAmountsMember2020-04-012021-03-310001004315nggtf:NGVenturesAndOtherMember2020-04-012021-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMember2019-04-012020-03-310001004315nggtf:NGVenturesAndOtherMemberifrs-full:EliminationOfIntersegmentAmountsMember2019-04-012020-03-310001004315nggtf:NGVenturesAndOtherMember2019-04-012020-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMember2018-04-012019-03-310001004315nggtf:NGVenturesAndOtherMemberifrs-full:EliminationOfIntersegmentAmountsMember2018-04-012019-03-310001004315nggtf:NGVenturesAndOtherMember2018-04-012019-03-310001004315nggtf:OperatingSegmentsAndUnallocatedAmountsMember2020-04-012021-03-310001004315ifrs-full:EliminationOfIntersegmentAmountsMember2020-04-012021-03-310001004315nggtf:OperatingSegmentsAndUnallocatedAmountsMember2019-04-012020-03-310001004315ifrs-full:EliminationOfIntersegmentAmountsMember2019-04-012020-03-310001004315nggtf:OperatingSegmentsAndUnallocatedAmountsMember2018-04-012019-03-310001004315ifrs-full:EliminationOfIntersegmentAmountsMember2018-04-012019-03-310001004315ifrs-full:CountryOfDomicileMember2020-04-012021-03-310001004315ifrs-full:CountryOfDomicileMember2019-04-012020-03-310001004315ifrs-full:CountryOfDomicileMember2018-04-012019-03-310001004315ifrs-full:ForeignCountriesMember2020-04-012021-03-310001004315ifrs-full:ForeignCountriesMember2019-04-012020-03-310001004315ifrs-full:ForeignCountriesMember2018-04-012019-03-310001004315nggtf:NGVenturesMemberifrs-full:UnallocatedAmountsMember2020-04-012021-03-310001004315nggtf:NGVenturesMemberifrs-full:UnallocatedAmountsMember2019-04-012020-03-310001004315nggtf:NGVenturesMemberifrs-full:UnallocatedAmountsMember2018-04-012019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:BeforeExceptionalItemsAndRemeasurementsMembernggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:BeforeExceptionalItemsAndRemeasurementsMembernggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:BeforeExceptionalItemsAndRemeasurementsMembernggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2020-04-012021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2019-04-012020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2018-04-012019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2020-04-012021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2019-04-012020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2018-04-012019-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:BeforeExceptionalItemsAndRemeasurementsMembernggtf:NGVenturesAndOtherMember2020-04-012021-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:BeforeExceptionalItemsAndRemeasurementsMembernggtf:NGVenturesAndOtherMember2019-04-012020-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:BeforeExceptionalItemsAndRemeasurementsMembernggtf:NGVenturesAndOtherMember2018-04-012019-03-310001004315nggtf:BeforeExceptionalItemsAndRemeasurementsMember2020-04-012021-03-310001004315nggtf:BeforeExceptionalItemsAndRemeasurementsMember2019-04-012020-03-310001004315nggtf:BeforeExceptionalItemsAndRemeasurementsMember2018-04-012019-03-310001004315ifrs-full:CountryOfDomicileMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2020-04-012021-03-310001004315ifrs-full:CountryOfDomicileMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2019-04-012020-03-310001004315ifrs-full:CountryOfDomicileMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2018-04-012019-03-310001004315ifrs-full:ForeignCountriesMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2020-04-012021-03-310001004315ifrs-full:ForeignCountriesMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2019-04-012020-03-310001004315ifrs-full:ForeignCountriesMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2018-04-012019-03-310001004315nggtf:ExceptionalItemsAndRemeasurementsMember2020-04-012021-03-310001004315nggtf:ExceptionalItemsAndRemeasurementsMember2019-04-012020-03-310001004315nggtf:ExceptionalItemsAndRemeasurementsMember2018-04-012019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:ExceptionalItemsAndRemeasurementsMembernggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:ExceptionalItemsAndRemeasurementsMembernggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:ExceptionalItemsAndRemeasurementsMembernggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMembernggtf:ExceptionalItemsAndRemeasurementsMember2020-04-012021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMembernggtf:ExceptionalItemsAndRemeasurementsMember2019-04-012020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMembernggtf:ExceptionalItemsAndRemeasurementsMember2018-04-012019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMembernggtf:ExceptionalItemsAndRemeasurementsMember2020-04-012021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMembernggtf:ExceptionalItemsAndRemeasurementsMember2019-04-012020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMembernggtf:ExceptionalItemsAndRemeasurementsMember2018-04-012019-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMembernggtf:ExceptionalItemsAndRemeasurementsMember2020-04-012021-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMembernggtf:ExceptionalItemsAndRemeasurementsMember2019-04-012020-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMembernggtf:ExceptionalItemsAndRemeasurementsMember2018-04-012019-03-310001004315nggtf:NGVenturesMemberifrs-full:UnallocatedAmountsMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2020-04-012021-03-310001004315nggtf:NGVenturesMemberifrs-full:UnallocatedAmountsMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2019-04-012020-03-310001004315nggtf:NGVenturesMemberifrs-full:UnallocatedAmountsMembernggtf:BeforeExceptionalItemsAndRemeasurementsMember2018-04-012019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKElectricityTransmissionMember2021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKElectricityTransmissionMember2020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKElectricityTransmissionMember2019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMember2021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMember2020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:UKGasTransmissionMember2019-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMember2021-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMember2020-03-310001004315ifrs-full:OperatingSegmentsMembernggtf:USRegulatedMember2019-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMember2021-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMember2020-03-310001004315ifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMember2019-03-310001004315ifrs-full:CountryOfDomicileMember2021-03-310001004315ifrs-full:CountryOfDomicileMember2020-03-310001004315ifrs-full:CountryOfDomicileMember2019-03-310001004315ifrs-full:ForeignCountriesMember2021-03-310001004315ifrs-full:ForeignCountriesMember2020-03-310001004315ifrs-full:ForeignCountriesMember2019-03-310001004315nggtf:PropertyPlantandEquipment1Member2021-03-310001004315nggtf:PropertyPlantandEquipment1Member2020-03-310001004315nggtf:PropertyPlantandEquipment1Member2019-03-310001004315nggtf:PropertyPlantandEquipment1Member2020-04-012021-03-310001004315nggtf:PropertyPlantandEquipment1Member2019-04-012020-03-310001004315nggtf:PropertyPlantandEquipment1Member2018-04-012019-03-310001004315nggtf:IntangibleAssetsMember2021-03-310001004315nggtf:IntangibleAssetsMember2020-03-310001004315nggtf:IntangibleAssetsMember2019-03-310001004315nggtf:IntangibleAssetsMember2020-04-012021-03-310001004315nggtf:IntangibleAssetsMember2019-04-012020-03-310001004315nggtf:IntangibleAssetsMember2018-04-012019-03-310001004315nggtf:A2020SoftwareAssetsAndPropertiesTransferFromNGVAndOtherSegmentToUSRegulatedSegmentRemaininginNGVenturesAndOtherMemberifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMember2019-03-310001004315nggtf:A2020SoftwareAssetsAndPropertiesTransferFromNGVAndOtherSegmentToUSRegulatedSegmentRemaininginNGVenturesAndOtherMemberifrs-full:UnallocatedAmountsMembernggtf:NGVenturesAndOtherMember2018-04-012019-03-310001004315nggtf:NGVenturesMemberifrs-full:UnallocatedAmountsMember2021-03-310001004315nggtf:NGVenturesMemberifrs-full:UnallocatedAmountsMember2020-03-310001004315nggtf:NGVenturesMemberifrs-full:UnallocatedAmountsMember2019-03-310001004315nggtf:UKElectricityTransmissionMembernggtf:TransmissionMember2020-04-012021-03-310001004315nggtf:UKGasTransmissionMembernggtf:TransmissionMember2020-04-012021-03-310001004315nggtf:USRegulatedMembernggtf:TransmissionMember2020-04-012021-03-310001004315nggtf:NGVandOtherMembernggtf:TransmissionMember2020-04-012021-03-310001004315nggtf:TransmissionMember2020-04-012021-03-310001004315nggtf:DistributionMembernggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315nggtf:UKGasTransmissionMembernggtf:DistributionMember2020-04-012021-03-310001004315nggtf:USRegulatedMembernggtf:DistributionMember2020-04-012021-03-310001004315nggtf:DistributionMembernggtf:NGVandOtherMember2020-04-012021-03-310001004315nggtf:DistributionMember2020-04-012021-03-310001004315nggtf:SystemOperatorMembernggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315nggtf:UKGasTransmissionMembernggtf:SystemOperatorMember2020-04-012021-03-310001004315nggtf:USRegulatedMembernggtf:SystemOperatorMember2020-04-012021-03-310001004315nggtf:SystemOperatorMembernggtf:NGVandOtherMember2020-04-012021-03-310001004315nggtf:SystemOperatorMember2020-04-012021-03-310001004315nggtf:OtherMembernggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315nggtf:UKGasTransmissionMembernggtf:OtherMember2020-04-012021-03-310001004315nggtf:USRegulatedMembernggtf:OtherMember2020-04-012021-03-310001004315nggtf:NGVandOtherMembernggtf:OtherMember2020-04-012021-03-310001004315nggtf:OtherMember2020-04-012021-03-310001004315nggtf:NGVandOtherMember2020-04-012021-03-310001004315nggtf:GenerationMembernggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315nggtf:UKGasTransmissionMembernggtf:GenerationMember2020-04-012021-03-310001004315nggtf:USRegulatedMembernggtf:GenerationMember2020-04-012021-03-310001004315nggtf:GenerationMembernggtf:NGVandOtherMember2020-04-012021-03-310001004315nggtf:GenerationMember2020-04-012021-03-310001004315ifrs-full:CountryOfDomicileMembernggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315nggtf:UKGasTransmissionMemberifrs-full:CountryOfDomicileMember2020-04-012021-03-310001004315nggtf:USRegulatedMemberifrs-full:CountryOfDomicileMember2020-04-012021-03-310001004315ifrs-full:CountryOfDomicileMembernggtf:NGVandOtherMember2020-04-012021-03-310001004315country:USnggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315nggtf:UKGasTransmissionMembercountry:US2020-04-012021-03-310001004315nggtf:USRegulatedMembercountry:US2020-04-012021-03-310001004315nggtf:NGVandOtherMembercountry:US2020-04-012021-03-310001004315country:US2020-04-012021-03-310001004315nggtf:UKElectricityTransmissionMembernggtf:TransmissionMember2019-04-012020-03-310001004315nggtf:UKGasTransmissionMembernggtf:TransmissionMember2019-04-012020-03-310001004315nggtf:USRegulatedMembernggtf:TransmissionMember2019-04-012020-03-310001004315nggtf:NGVandOtherMembernggtf:TransmissionMember2019-04-012020-03-310001004315nggtf:TransmissionMember2019-04-012020-03-310001004315nggtf:DistributionMembernggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315nggtf:UKGasTransmissionMembernggtf:DistributionMember2019-04-012020-03-310001004315nggtf:USRegulatedMembernggtf:DistributionMember2019-04-012020-03-310001004315nggtf:DistributionMembernggtf:NGVandOtherMember2019-04-012020-03-310001004315nggtf:DistributionMember2019-04-012020-03-310001004315nggtf:SystemOperatorMembernggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315nggtf:UKGasTransmissionMembernggtf:SystemOperatorMember2019-04-012020-03-310001004315nggtf:USRegulatedMembernggtf:SystemOperatorMember2019-04-012020-03-310001004315nggtf:SystemOperatorMembernggtf:NGVandOtherMember2019-04-012020-03-310001004315nggtf:SystemOperatorMember2019-04-012020-03-310001004315nggtf:OtherMembernggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315nggtf:UKGasTransmissionMembernggtf:OtherMember2019-04-012020-03-310001004315nggtf:USRegulatedMembernggtf:OtherMember2019-04-012020-03-310001004315nggtf:NGVandOtherMembernggtf:OtherMember2019-04-012020-03-310001004315nggtf:OtherMember2019-04-012020-03-310001004315nggtf:NGVandOtherMember2019-04-012020-03-310001004315nggtf:GenerationMembernggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315nggtf:UKGasTransmissionMembernggtf:GenerationMember2019-04-012020-03-310001004315nggtf:USRegulatedMembernggtf:GenerationMember2019-04-012020-03-310001004315nggtf:GenerationMembernggtf:NGVandOtherMember2019-04-012020-03-310001004315nggtf:GenerationMember2019-04-012020-03-310001004315ifrs-full:CountryOfDomicileMembernggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315nggtf:UKGasTransmissionMemberifrs-full:CountryOfDomicileMember2019-04-012020-03-310001004315nggtf:USRegulatedMemberifrs-full:CountryOfDomicileMember2019-04-012020-03-310001004315ifrs-full:CountryOfDomicileMembernggtf:NGVandOtherMember2019-04-012020-03-310001004315country:USnggtf:UKElectricityTransmissionMember2019-04-012020-03-310001004315nggtf:UKGasTransmissionMembercountry:US2019-04-012020-03-310001004315nggtf:USRegulatedMembercountry:US2019-04-012020-03-310001004315nggtf:NGVandOtherMembercountry:US2019-04-012020-03-310001004315country:US2019-04-012020-03-310001004315nggtf:UKElectricityTransmissionMembernggtf:TransmissionMember2018-04-012019-03-310001004315nggtf:UKGasTransmissionMembernggtf:TransmissionMember2018-04-012019-03-310001004315nggtf:USRegulatedMembernggtf:TransmissionMember2018-04-012019-03-310001004315nggtf:NGVandOtherMembernggtf:TransmissionMember2018-04-012019-03-310001004315nggtf:TransmissionMember2018-04-012019-03-310001004315nggtf:DistributionMembernggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315nggtf:UKGasTransmissionMembernggtf:DistributionMember2018-04-012019-03-310001004315nggtf:USRegulatedMembernggtf:DistributionMember2018-04-012019-03-310001004315nggtf:DistributionMembernggtf:NGVandOtherMember2018-04-012019-03-310001004315nggtf:DistributionMember2018-04-012019-03-310001004315nggtf:SystemOperatorMembernggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315nggtf:UKGasTransmissionMembernggtf:SystemOperatorMember2018-04-012019-03-310001004315nggtf:USRegulatedMembernggtf:SystemOperatorMember2018-04-012019-03-310001004315nggtf:SystemOperatorMembernggtf:NGVandOtherMember2018-04-012019-03-310001004315nggtf:SystemOperatorMember2018-04-012019-03-310001004315nggtf:OtherMembernggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315nggtf:UKGasTransmissionMembernggtf:OtherMember2018-04-012019-03-310001004315nggtf:USRegulatedMembernggtf:OtherMember2018-04-012019-03-310001004315nggtf:NGVandOtherMembernggtf:OtherMember2018-04-012019-03-310001004315nggtf:OtherMember2018-04-012019-03-310001004315nggtf:NGVandOtherMember2018-04-012019-03-310001004315nggtf:GenerationMembernggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315nggtf:UKGasTransmissionMembernggtf:GenerationMember2018-04-012019-03-310001004315nggtf:USRegulatedMembernggtf:GenerationMember2018-04-012019-03-310001004315nggtf:GenerationMembernggtf:NGVandOtherMember2018-04-012019-03-310001004315nggtf:GenerationMember2018-04-012019-03-310001004315ifrs-full:CountryOfDomicileMembernggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315nggtf:UKGasTransmissionMemberifrs-full:CountryOfDomicileMember2018-04-012019-03-310001004315nggtf:USRegulatedMemberifrs-full:CountryOfDomicileMember2018-04-012019-03-310001004315ifrs-full:CountryOfDomicileMembernggtf:NGVandOtherMember2018-04-012019-03-310001004315country:USnggtf:UKElectricityTransmissionMember2018-04-012019-03-310001004315nggtf:UKGasTransmissionMembercountry:US2018-04-012019-03-310001004315nggtf:USRegulatedMembercountry:US2018-04-012019-03-310001004315nggtf:NGVandOtherMembercountry:US2018-04-012019-03-310001004315country:US2018-04-012019-03-310001004315nggtf:ConnectionsMembernggtf:UKElectricityTransmissionMember2020-04-012021-03-310001004315nggtf:UKGasTransmissionMembernggtf:ConnectionsMember2020-04-012021-03-310001004315nggtf:ConnectionsMembernggtf:NGVandOtherMember2020-04-012021-03-310001004315nggtf:USRegulatedMembernggtf:ConnectionsMember2020-04-012021-03-310001004315nggtf:ConnectionsMemberifrs-full:WeightedAverageMember2020-04-012021-03-310001004315nggtf:UKElectricityTransmissionMember2021-03-310001004315nggtf:UKElectricityTransmissionMember2020-03-310001004315nggtf:UKElectricityTransmissionMember2019-03-310001004315nggtf:NationalGridGrainLNGLimitedMember2021-03-310001004315nggtf:NationalGridGrainLNGLimitedMember2020-03-310001004315nggtf:NationalGridGrainLNGLimitedMember2019-03-31nggtf:employee0001004315country:US2021-03-310001004315country:US2020-03-310001004315country:US2019-03-310001004315nggtf:ExceptionalItemsMember2020-04-012021-03-310001004315nggtf:ExceptionalItemsMember2019-04-012020-03-310001004315nggtf:ExceptionalItemsMember2018-04-012019-03-310001004315nggtf:RemeasurementsMembernggtf:CommodityDerivativesMember2020-04-012021-03-310001004315nggtf:RemeasurementsMembernggtf:CommodityDerivativesMember2019-04-012020-03-310001004315nggtf:RemeasurementsMembernggtf:CommodityDerivativesMember2018-04-012019-03-310001004315nggtf:RemeasurementsMember2020-04-012021-03-310001004315nggtf:RemeasurementsMember2019-04-012020-03-310001004315nggtf:RemeasurementsMember2018-04-012019-03-310001004315country:GBnggtf:ExceptionalItemsMember2020-04-012021-03-310001004315country:GBnggtf:ExceptionalItemsMember2019-04-012020-03-310001004315country:GBnggtf:ExceptionalItemsMember2018-04-012019-03-31nggtf:numberOfBusinessUnits0001004315ifrs-full:OtherEnvironmentRelatedProvisionMembercountry:US2021-03-310001004315country:GB2019-04-012020-03-310001004315nggtf:ExceptionalItemsChangeInEstimateOfTotalCostAndCostSharingMemberifrs-full:OtherEnvironmentRelatedProvisionMember2019-04-012020-03-310001004315ifrs-full:OtherEnvironmentRelatedProvisionMembernggtf:ExceptionalItemsImpactOfChangeInTheDiscountRateMember2019-04-012020-03-310001004315ifrs-full:OtherEnvironmentRelatedProvisionMembercountry:US2019-04-012020-03-310001004315country:GBifrs-full:OtherEnvironmentRelatedProvisionMember2019-04-012020-03-310001004315ifrs-full:OtherEnvironmentRelatedProvisionMember2019-04-012020-03-31nggtf:cancellationnggtf:settlement0001004315stpr:MAnggtf:USRegulatedMember2018-05-310001004315nggtf:ExceptionalItemsMember2016-04-012017-03-310001004315nggtf:DomesticTaxAuthority1Member2020-04-012021-03-310001004315nggtf:DomesticTaxAuthority1Member2019-04-012020-03-310001004315nggtf:DomesticTaxAuthority1Member2018-04-012019-03-310001004315nggtf:ForeignTaxAuthority1Member2020-04-012021-03-310001004315nggtf:ForeignTaxAuthority1Member2019-04-012020-03-310001004315nggtf:ForeignTaxAuthority1Member2018-04-012019-03-310001004315ifrs-full:ChangesInTaxRatesOrTaxLawsEnactedOrAnnouncedMembersrt:ScenarioForecastMember2020-04-012021-03-310001004315nggtf:AcceleratedTaxDepreciationRelatedTemporaryDifferencesMember2019-03-310001004315nggtf:SharebasedPaymentsRelatedTemporaryDifferencesMember2019-03-310001004315nggtf:PensionsAndOtherPostretirementBenefitsRelatedTemporaryDifferencesMember2019-03-310001004315nggtf:FinancialInstrumentsRelatedTemporaryDifferencesMember2019-03-310001004315ifrs-full:OtherTemporaryDifferencesMember2019-03-310001004315nggtf:AcceleratedTaxDepreciationRelatedTemporaryDifferencesMember2019-04-012020-03-310001004315nggtf:SharebasedPaymentsRelatedTemporaryDifferencesMember2019-04-012020-03-310001004315nggtf:PensionsAndOtherPostretirementBenefitsRelatedTemporaryDifferencesMember2019-04-012020-03-310001004315nggtf:FinancialInstrumentsRelatedTemporaryDifferencesMember2019-04-012020-03-310001004315ifrs-full:OtherTemporaryDifferencesMember2019-04-012020-03-310001004315nggtf:AcceleratedTaxDepreciationRelatedTemporaryDifferencesMember2020-03-310001004315nggtf:SharebasedPaymentsRelatedTemporaryDifferencesMember2020-03-310001004315nggtf:PensionsAndOtherPostretirementBenefitsRelatedTemporaryDifferencesMember2020-03-310001004315nggtf:FinancialInstrumentsRelatedTemporaryDifferencesMember2020-03-310001004315ifrs-full:OtherTemporaryDifferencesMember2020-03-310001004315nggtf:AcceleratedTaxDepreciationRelatedTemporaryDifferencesMember2020-04-012021-03-310001004315nggtf:SharebasedPaymentsRelatedTemporaryDifferencesMember2020-04-012021-03-310001004315nggtf:PensionsAndOtherPostretirementBenefitsRelatedTemporaryDifferencesMember2020-04-012021-03-310001004315nggtf:FinancialInstrumentsRelatedTemporaryDifferencesMember2020-04-012021-03-310001004315ifrs-full:OtherTemporaryDifferencesMember2020-04-012021-03-310001004315nggtf:AcceleratedTaxDepreciationRelatedTemporaryDifferencesMember2021-03-310001004315nggtf:SharebasedPaymentsRelatedTemporaryDifferencesMember2021-03-310001004315nggtf:PensionsAndOtherPostretirementBenefitsRelatedTemporaryDifferencesMember2021-03-310001004315nggtf:FinancialInstrumentsRelatedTemporaryDifferencesMember2021-03-310001004315ifrs-full:OtherTemporaryDifferencesMember2021-03-310001004315ifrs-full:UnusedTaxLossesMember2021-03-310001004315ifrs-full:UnusedTaxLossesMember2020-03-310001004315nggtf:TemporaryDifferencesEnvironmentalProvisionsMember2021-03-310001004315nggtf:TemporaryDifferencesEnvironmentalProvisionsMember2020-03-310001004315ifrs-full:AllowanceForCreditLossesMember2021-03-310001004315ifrs-full:AllowanceForCreditLossesMember2020-03-310001004315nggtf:ReclassificationFromOtherTemporaryDifferencesToSharebasedPaymentsMembernggtf:SharebasedPaymentsRelatedTemporaryDifferencesMember2020-04-012021-03-310001004315nggtf:CapitalLossesMember2021-03-310001004315nggtf:CapitalLossesMember2020-03-310001004315nggtf:NonTradeDeficitsMember2021-03-310001004315nggtf:NonTradeDeficitsMember2020-03-310001004315nggtf:TradingLossesMember2021-03-310001004315nggtf:TradingLossesMember2020-03-310001004315nggtf:InterimDividendinRespectofCurrentYearMember2020-04-012021-03-310001004315nggtf:InterimDividendinRespectofCurrentYearMember2019-04-012020-03-310001004315nggtf:InterimDividendinRespectofCurrentYearMember2018-04-012019-03-310001004315nggtf:FinalDividendinRespectofPriorYearMember2020-04-012021-03-310001004315nggtf:FinalDividendinRespectofPriorYearMember2019-04-012020-03-310001004315nggtf:FinalDividendinRespectofPriorYearMember2018-04-012019-03-310001004315nggtf:TheNarragansettElectricCompanyMember2021-03-172021-03-170001004315srt:ScenarioForecastMembernggtf:TheNarragansettElectricCompanyMember2022-03-012022-03-31iso4217:USD0001004315ifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleMemberifrs-full:GoodwillMember2021-03-310001004315ifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleMemberifrs-full:IntangibleAssetsOtherThanGoodwillMember2021-03-310001004315ifrs-full:PropertyPlantAndEquipmentMemberifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleMember2021-03-310001004315ifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleMemberifrs-full:TradeReceivablesMember2021-03-310001004315ifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleMembernggtf:CashandCashEquivalentsMember2021-03-310001004315ifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleMemberifrs-full:OtherAssetsMember2021-03-310001004315ifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleMember2021-03-310001004315ifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleMembernggtf:BorrowingsMember2021-03-310001004315ifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleMembernggtf:PensionLiabilitiesMember2021-03-310001004315ifrs-full:AssetsAndLiabilitiesClassifiedAsHeldForSaleMembernggtf:OtherLiabilitiesMember2021-03-310001004315nggtf:TheNarragansettElectricCompanyMember2020-04-012021-03-310001004315nggtf:TheNarragansettElectricCompanyMember2019-04-012020-03-310001004315nggtf:TheNarragansettElectricCompanyMember2018-04-012019-03-310001004315nggtf:QuadgasHoldcoLimitedCadentMember2019-06-012019-06-300001004315nggtf:QuadgasHoldCoLimitedMember2019-06-012019-06-300001004315nggtf:QuadgasHoldCoLimitedMember2019-06-300001004315nggtf:QuadgasHoldCoLimitedMemberifrs-full:DiscontinuedOperationsMember2019-06-012019-06-300001004315ifrs-full:DiscontinuedOperationsMember2019-06-012019-06-300001004315nggtf:QuadgasHoldCoLimitedMemberifrs-full:DiscontinuedOperationsMember2018-04-012019-03-310001004315ifrs-full:GoodwillMember2019-03-310001004315ifrs-full:GoodwillMember2019-04-012020-03-310001004315ifrs-full:GoodwillMember2020-03-310001004315ifrs-full:GoodwillMember2020-04-012021-03-310001004315ifrs-full:GoodwillMember2021-03-310001004315ifrs-full:GoodwillMemberifrs-full:AccumulatedImpairmentMember2021-03-310001004315ifrs-full:GoodwillMemberifrs-full:AccumulatedImpairmentMember2020-03-310001004315nggtf:NewYorkCGUMemberifrs-full:GoodwillMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315nggtf:NewYorkCGUMemberifrs-full:GoodwillMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:GoodwillMembernggtf:MassachusettsCGUMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315ifrs-full:GoodwillMembernggtf:MassachusettsCGUMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:GoodwillMembernggtf:RhodeIslandCGUMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315ifrs-full:GoodwillMembernggtf:RhodeIslandCGUMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315nggtf:FederalCGUMemberifrs-full:GoodwillMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315nggtf:FederalCGUMemberifrs-full:GoodwillMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315nggtf:NationalGridRenewablesCGUMemberifrs-full:GoodwillMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315nggtf:NationalGridRenewablesCGUMemberifrs-full:GoodwillMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:ComputerSoftwareMember2020-04-012021-03-310001004315ifrs-full:ComputerSoftwareMemberifrs-full:TopOfRangeMember2020-04-012021-03-310001004315ifrs-full:ComputerSoftwareMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315ifrs-full:ConstructionInProgressMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315ifrs-full:GrossCarryingAmountMember2019-03-310001004315ifrs-full:ComputerSoftwareMemberifrs-full:GrossCarryingAmountMember2019-04-012020-03-310001004315ifrs-full:ConstructionInProgressMemberifrs-full:GrossCarryingAmountMember2019-04-012020-03-310001004315ifrs-full:GrossCarryingAmountMember2019-04-012020-03-310001004315ifrs-full:ComputerSoftwareMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:ConstructionInProgressMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:ComputerSoftwareMemberifrs-full:GrossCarryingAmountMember2020-04-012021-03-310001004315ifrs-full:ConstructionInProgressMemberifrs-full:GrossCarryingAmountMember2020-04-012021-03-310001004315ifrs-full:GrossCarryingAmountMember2020-04-012021-03-310001004315ifrs-full:ComputerSoftwareMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315ifrs-full:ConstructionInProgressMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315ifrs-full:GrossCarryingAmountMember2021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ComputerSoftwareMember2019-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ConstructionInProgressMember2019-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMember2019-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ComputerSoftwareMember2019-04-012020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ConstructionInProgressMember2019-04-012020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMember2019-04-012020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ComputerSoftwareMember2020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ConstructionInProgressMember2020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMember2020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ComputerSoftwareMember2020-04-012021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ConstructionInProgressMember2020-04-012021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMember2020-04-012021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ComputerSoftwareMember2021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ConstructionInProgressMember2021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMember2021-03-310001004315ifrs-full:ComputerSoftwareMember2021-03-310001004315ifrs-full:ConstructionInProgressMember2021-03-310001004315ifrs-full:ComputerSoftwareMember2020-03-310001004315ifrs-full:ConstructionInProgressMember2020-03-310001004315nggtf:GasBusinessEnablementSystemMember2021-03-310001004315nggtf:GasBusinessEnablementSystemMember2020-03-310001004315nggtf:GasBusinessEnablementSystemMember2020-04-012021-03-310001004315nggtf:UKGeneralLedgerSystemMember2021-03-310001004315nggtf:UKGeneralLedgerSystemMember2020-03-310001004315ifrs-full:LandAndBuildingsMemberifrs-full:TopOfRangeMembercountry:GB2020-04-012021-03-310001004315ifrs-full:LandAndBuildingsMemberifrs-full:TopOfRangeMembercountry:US2020-04-012021-03-310001004315ifrs-full:LandAndBuildingsMemberifrs-full:WeightedAverageMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembercountry:GBnggtf:ElectricityTransmissionPlantMember2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembercountry:GBnggtf:ElectricityTransmissionPlantMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembernggtf:ElectricityTransmissionPlantMembercountry:US2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembernggtf:ElectricityTransmissionPlantMembercountry:US2020-04-012021-03-310001004315nggtf:ElectricityTransmissionPlantMemberifrs-full:WeightedAverageMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembernggtf:ElectricityDistributionPlantMembercountry:US2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembernggtf:ElectricityDistributionPlantMembercountry:US2020-04-012021-03-310001004315nggtf:ElectricityDistributionPlantMemberifrs-full:WeightedAverageMember2020-04-012021-03-310001004315nggtf:ElectricityGenerationPlantMemberifrs-full:BottomOfRangeMembercountry:US2020-04-012021-03-310001004315nggtf:ElectricityGenerationPlantMemberifrs-full:TopOfRangeMembercountry:US2020-04-012021-03-310001004315nggtf:ElectricityGenerationPlantMemberifrs-full:WeightedAverageMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembercountry:GBnggtf:InterconnectorPlantMember2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembercountry:GBnggtf:InterconnectorPlantMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembernggtf:InterconnectorPlantMembercountry:US2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembernggtf:InterconnectorPlantMembercountry:US2020-04-012021-03-310001004315nggtf:InterconnectorPlantMemberifrs-full:WeightedAverageMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembercountry:GBnggtf:GasPlantMainsServicesAndRegulatingEquipmentMember2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembercountry:GBnggtf:GasPlantMainsServicesAndRegulatingEquipmentMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembercountry:USnggtf:GasPlantMainsServicesAndRegulatingEquipmentMember2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembercountry:USnggtf:GasPlantMainsServicesAndRegulatingEquipmentMember2020-04-012021-03-310001004315ifrs-full:WeightedAverageMembernggtf:GasPlantMainsServicesAndRegulatingEquipmentMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembercountry:GBnggtf:GasPlantStorageMember2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembercountry:GBnggtf:GasPlantStorageMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembercountry:USnggtf:GasPlantStorageMember2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembercountry:USnggtf:GasPlantStorageMember2020-04-012021-03-310001004315ifrs-full:WeightedAverageMembernggtf:GasPlantStorageMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembercountry:GBnggtf:GasPlantMetersMember2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembercountry:GBnggtf:GasPlantMetersMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMembercountry:USnggtf:GasPlantMetersMember2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembercountry:USnggtf:GasPlantMetersMember2020-04-012021-03-310001004315ifrs-full:WeightedAverageMembernggtf:GasPlantMetersMember2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembercountry:GBnggtf:MotorVehiclesAndOfficeEquipmentMember2020-04-012021-03-310001004315ifrs-full:TopOfRangeMembercountry:USnggtf:MotorVehiclesAndOfficeEquipmentMember2020-04-012021-03-310001004315ifrs-full:WeightedAverageMembernggtf:MotorVehiclesAndOfficeEquipmentMember2020-04-012021-03-310001004315nggtf:UKRegulatedMember2020-04-012021-03-310001004315nggtf:UKRegulatedMember2019-04-012020-03-310001004315ifrs-full:LandAndBuildingsMemberifrs-full:PreviouslyStatedMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315ifrs-full:PreviouslyStatedMembernggtf:PlantAndMachineryMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315ifrs-full:PreviouslyStatedMemberifrs-full:GrossCarryingAmountMemberifrs-full:ConstructionInProgressMember2019-03-310001004315ifrs-full:PreviouslyStatedMembernggtf:MotorVehiclesAndOfficeEquipmentMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315ifrs-full:PreviouslyStatedMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315ifrs-full:LandAndBuildingsMembernggtf:IFRS16Memberifrs-full:GrossCarryingAmountMember2019-03-310001004315nggtf:IFRS16Membernggtf:PlantAndMachineryMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315nggtf:IFRS16Memberifrs-full:GrossCarryingAmountMemberifrs-full:ConstructionInProgressMember2019-03-310001004315nggtf:IFRS16Membernggtf:MotorVehiclesAndOfficeEquipmentMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315nggtf:IFRS16Memberifrs-full:GrossCarryingAmountMember2019-03-310001004315ifrs-full:LandAndBuildingsMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315nggtf:PlantAndMachineryMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315ifrs-full:GrossCarryingAmountMemberifrs-full:ConstructionInProgressMember2019-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMemberifrs-full:GrossCarryingAmountMember2019-03-310001004315ifrs-full:LandAndBuildingsMemberifrs-full:GrossCarryingAmountMember2019-04-012020-03-310001004315nggtf:PlantAndMachineryMemberifrs-full:GrossCarryingAmountMember2019-04-012020-03-310001004315ifrs-full:GrossCarryingAmountMemberifrs-full:ConstructionInProgressMember2019-04-012020-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMemberifrs-full:GrossCarryingAmountMember2019-04-012020-03-310001004315ifrs-full:LandAndBuildingsMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315nggtf:PlantAndMachineryMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:GrossCarryingAmountMemberifrs-full:ConstructionInProgressMember2020-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:LandAndBuildingsMemberifrs-full:GrossCarryingAmountMember2020-04-012021-03-310001004315nggtf:PlantAndMachineryMemberifrs-full:GrossCarryingAmountMember2020-04-012021-03-310001004315ifrs-full:GrossCarryingAmountMemberifrs-full:ConstructionInProgressMember2020-04-012021-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMemberifrs-full:GrossCarryingAmountMember2020-04-012021-03-310001004315ifrs-full:LandAndBuildingsMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315nggtf:PlantAndMachineryMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315ifrs-full:GrossCarryingAmountMemberifrs-full:ConstructionInProgressMember2021-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:LandAndBuildingsMember2019-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMembernggtf:PlantAndMachineryMember2019-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ConstructionInProgressMember2019-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMembernggtf:MotorVehiclesAndOfficeEquipmentMember2019-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:LandAndBuildingsMember2019-04-012020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMembernggtf:PlantAndMachineryMember2019-04-012020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ConstructionInProgressMember2019-04-012020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMembernggtf:MotorVehiclesAndOfficeEquipmentMember2019-04-012020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:LandAndBuildingsMember2020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMembernggtf:PlantAndMachineryMember2020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ConstructionInProgressMember2020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMembernggtf:MotorVehiclesAndOfficeEquipmentMember2020-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:LandAndBuildingsMember2020-04-012021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMembernggtf:PlantAndMachineryMember2020-04-012021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ConstructionInProgressMember2020-04-012021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMembernggtf:MotorVehiclesAndOfficeEquipmentMember2020-04-012021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:LandAndBuildingsMember2021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMembernggtf:PlantAndMachineryMember2021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMemberifrs-full:ConstructionInProgressMember2021-03-310001004315ifrs-full:AccumulatedDepreciationAndAmortisationMembernggtf:MotorVehiclesAndOfficeEquipmentMember2021-03-310001004315ifrs-full:LandAndBuildingsMember2021-03-310001004315nggtf:PlantAndMachineryMember2021-03-310001004315ifrs-full:ConstructionInProgressMember2021-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMember2021-03-310001004315ifrs-full:LandAndBuildingsMember2020-03-310001004315nggtf:PlantAndMachineryMember2020-03-310001004315ifrs-full:ConstructionInProgressMember2020-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMember2020-03-310001004315nggtf:IFRS16Member2019-04-010001004315nggtf:PropertyPlantandEquipment1Memberifrs-full:GrossCarryingAmountMember2020-04-012021-03-310001004315nggtf:PropertyPlantandEquipment1Memberifrs-full:GrossCarryingAmountMember2019-04-012020-03-310001004315nggtf:TradeAndOtherPayablesMember2021-03-310001004315nggtf:TradeAndOtherPayablesMember2020-03-310001004315nggtf:NoncurrentLiabilitiesMember2021-03-310001004315nggtf:NoncurrentLiabilitiesMember2020-03-310001004315nggtf:ContractliabilitiescurrentMember2021-03-310001004315nggtf:ContractliabilitiescurrentMember2020-03-310001004315nggtf:ContractliabilitiesnoncurrentMember2021-03-310001004315nggtf:ContractliabilitiesnoncurrentMember2020-03-310001004315ifrs-full:LandAndBuildingsMember2021-03-310001004315nggtf:PlantAndMachineryMember2021-03-310001004315ifrs-full:ConstructionInProgressMember2021-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMember2021-03-310001004315ifrs-full:LandAndBuildingsMember2020-04-012021-03-310001004315nggtf:PlantAndMachineryMember2020-04-012021-03-310001004315ifrs-full:ConstructionInProgressMember2020-04-012021-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMember2020-04-012021-03-310001004315ifrs-full:LandAndBuildingsMember2020-03-310001004315nggtf:PlantAndMachineryMember2020-03-310001004315ifrs-full:ConstructionInProgressMember2020-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMember2020-03-310001004315ifrs-full:LandAndBuildingsMember2019-04-012020-03-310001004315nggtf:PlantAndMachineryMember2019-04-012020-03-310001004315ifrs-full:ConstructionInProgressMember2019-04-012020-03-310001004315nggtf:MotorVehiclesAndOfficeEquipmentMember2019-04-012020-03-310001004315nggtf:InvestmentsShortTermMoneyFundsMember2021-03-310001004315nggtf:InvestmentsShortTermMoneyFundsMember2020-03-310001004315nggtf:CorporateCapitalVenturesAndOtherMember2021-03-310001004315nggtf:CorporateCapitalVenturesAndOtherMember2020-03-310001004315nggtf:UnquotedEquityInvestmentsMember2021-03-310001004315nggtf:UnquotedEquityInvestmentsMember2020-03-310001004315nggtf:RestrictedBalancesCollateralMember2021-03-310001004315nggtf:RestrictedBalancesCollateralMember2020-03-310001004315nggtf:InsuranceCompanyAndNonQualifiedPlanInvestmentsMember2021-03-310001004315nggtf:InsuranceCompanyAndNonQualifiedPlanInvestmentsMember2020-03-310001004315nggtf:CashSurrenderValueOfLifeInsurancePoliciesMember2021-03-310001004315nggtf:CashSurrenderValueOfLifeInsurancePoliciesMember2020-03-310001004315nggtf:RestrictedBalancesOtherMember2021-03-310001004315nggtf:RestrictedBalancesOtherMember2020-03-310001004315nggtf:InvestmentsInsuranceCompanyFundMember2021-03-310001004315nggtf:InvestmentsInsuranceCompanyFundMember2020-03-310001004315ifrs-full:AssociatesMember2020-03-310001004315ifrs-full:JointVenturesMember2020-03-310001004315ifrs-full:AssociatesMember2019-03-310001004315ifrs-full:JointVenturesMember2019-03-310001004315ifrs-full:AssociatesMember2020-04-012021-03-310001004315ifrs-full:JointVenturesMember2020-04-012021-03-310001004315ifrs-full:AssociatesMember2019-04-012020-03-310001004315ifrs-full:JointVenturesMember2019-04-012020-03-310001004315ifrs-full:AssociatesMember2021-03-310001004315ifrs-full:JointVenturesMember2021-03-31nggtf:joint_venture0001004315nggtf:BritNedDevelopmentLimitedMember2020-04-012021-03-310001004315nggtf:NemoLinkLimitedMember2020-04-012021-03-310001004315nggtf:EmeraldEnergyVentureLLCMember2020-04-012021-03-310001004315nggtf:MillenniumPipelineCompanyLLCMember2020-04-012021-03-310001004315nggtf:BritNedDevelopmentLimitedMember2021-03-310001004315nggtf:BritNedDevelopmentLimitedMember2020-03-310001004315nggtf:MillenniumPipelineCompanyLLCMember2021-03-310001004315nggtf:MillenniumPipelineCompanyLLCMember2020-03-310001004315nggtf:NemoLinkLimitedMember2021-03-310001004315nggtf:NemoLinkLimitedMember2020-03-310001004315nggtf:EmeraldEnergyVentureLLCMember2021-03-310001004315nggtf:EmeraldEnergyVentureLLCMember2020-03-310001004315nggtf:BritNedDevelopmentLimitedMember2019-04-012020-03-310001004315nggtf:MillenniumPipelineCompanyLLCMember2019-04-012020-03-310001004315nggtf:NemoLinkLimitedMember2019-04-012020-03-310001004315nggtf:EmeraldEnergyVentureLLCMember2019-04-012020-03-310001004315nggtf:CurrentDerivativeFinancialAssetsMember2021-03-310001004315nggtf:CurrentDerivativeFinancialLiabilitiesMember2021-03-310001004315nggtf:CurrentDerivativeFinancialAssetsMember2020-03-310001004315nggtf:CurrentDerivativeFinancialLiabilitiesMember2020-03-310001004315nggtf:NonCurrentDerivativeFinancialAssetsMember2021-03-310001004315nggtf:NonCurrentDerivativeFinancialLiabilitiesMember2021-03-310001004315nggtf:NonCurrentDerivativeFinancialAssetsMember2020-03-310001004315nggtf:NonCurrentDerivativeFinancialLiabilitiesMember2020-03-310001004315nggtf:FinancingDerivativesMember2021-03-310001004315nggtf:FinancingDerivativesMember2020-03-310001004315nggtf:CommodityDerivativesMember2021-03-310001004315nggtf:CommodityDerivativesMember2020-03-310001004315nggtf:FinancingDerivativesInterestRateSwapsMember2021-03-310001004315nggtf:FinancingDerivativesInterestRateSwapsMember2020-03-310001004315nggtf:FinancingDerivativesCrossCurrencyInterestRateSwapsMember2021-03-310001004315nggtf:FinancingDerivativesCrossCurrencyInterestRateSwapsMember2020-03-310001004315nggtf:FinancingDerivativesForeignExchangeForwardContractsMember2021-03-310001004315nggtf:FinancingDerivativesForeignExchangeForwardContractsMember2020-03-310001004315nggtf:FinancingDerivativesInflationLinkedSwapsMember2021-03-310001004315nggtf:FinancingDerivativesInflationLinkedSwapsMember2020-03-310001004315nggtf:FinancingDerivativesEquityOptionsMember2021-03-310001004315nggtf:FinancingDerivativesEquityOptionsMember2020-03-310001004315nggtf:CapitalExpendituresMembernggtf:FinancingDerivativesForeignExchangeForwardContractsMember2021-03-310001004315nggtf:CapitalExpendituresMembernggtf:FinancingDerivativesForeignExchangeForwardContractsMember2020-03-310001004315nggtf:FinancingDerivativesDealContingentForeignExchangeForwardContractsMember2021-03-310001004315nggtf:FinancingDerivativesDealContingentForeignExchangeForwardContractsMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:NotLaterThanOneYearMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:NotLaterThanOneYearMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanFiveYearsMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanFiveYearsMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanOneYearMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:LaterThanOneYearMember2020-03-310001004315nggtf:FinancingDerivativesMembernggtf:LondonInterbankOfferedRateLIBOR1Member2021-03-310001004315nggtf:FinancingDerivativesMembernggtf:LondonInterbankOfferedRateGBPLIBORMember2021-03-310001004315nggtf:FinancingDerivativesMembernggtf:LondonInterbankOfferedRateUSDLIBORMember2021-03-310001004315nggtf:CommodityDerivativesForwardPurchasesGasMember2021-03-310001004315nggtf:CommodityDerivativesForwardPurchasesGasMember2020-03-310001004315nggtf:CommodityDerivativesElectricityCapacityMember2021-03-310001004315nggtf:CommodityDerivativesElectricitySwapsMember2021-03-310001004315nggtf:CommodityDerivativesElectricitySwapsMember2020-03-310001004315nggtf:CommodityDerivativesElectricityOptionsMember2021-03-310001004315nggtf:CommodityDerivativesGasSwapsMember2021-03-310001004315nggtf:CommodityDerivativesGasSwapsMember2020-03-310001004315nggtf:CommodityDerivativesGasOptionsMember2021-03-310001004315nggtf:CommodityDerivativesGasOptionsMember2020-03-310001004315ifrs-full:NotLaterThanOneYearMembernggtf:CommodityDerivativesMember2021-03-310001004315ifrs-full:NotLaterThanOneYearMembernggtf:CommodityDerivativesMember2020-03-310001004315ifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMembernggtf:CommodityDerivativesMember2021-03-310001004315ifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMembernggtf:CommodityDerivativesMember2020-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2021-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2020-03-310001004315ifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMembernggtf:CommodityDerivativesMember2021-03-310001004315ifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMembernggtf:CommodityDerivativesMember2020-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember2021-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember2020-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:LaterThanFiveYearsMember2021-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:LaterThanFiveYearsMember2020-03-310001004315ifrs-full:LaterThanOneYearMembernggtf:CommodityDerivativesMember2021-03-310001004315ifrs-full:LaterThanOneYearMembernggtf:CommodityDerivativesMember2020-03-31utr:MMBTUutr:GWh0001004315nggtf:CommodityDerivativesForwardPurchasesGasMember2020-04-012021-03-310001004315nggtf:CommodityDerivativesForwardPurchasesGasMember2019-04-012020-03-310001004315ifrs-full:TradeReceivablesMember2021-03-310001004315ifrs-full:TradeReceivablesMember2020-03-310001004315ifrs-full:TradeReceivablesMember2019-03-310001004315ifrs-full:TradeReceivablesMember2020-04-012021-03-310001004315ifrs-full:TradeReceivablesMember2019-04-012020-03-310001004315ifrs-full:TradeReceivablesMembercountry:GBifrs-full:GrossCarryingAmountMember2021-03-310001004315ifrs-full:TradeReceivablesMembercountry:USifrs-full:GrossCarryingAmountMember2021-03-310001004315ifrs-full:TradeReceivablesMemberifrs-full:GrossCarryingAmountMember2021-03-310001004315ifrs-full:TradeReceivablesMembercountry:GBifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:TradeReceivablesMembercountry:USifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:TradeReceivablesMemberifrs-full:GrossCarryingAmountMember2020-03-310001004315country:GBifrs-full:GrossCarryingAmountMember2021-03-310001004315country:USifrs-full:GrossCarryingAmountMember2021-03-310001004315country:GBifrs-full:GrossCarryingAmountMember2020-03-310001004315country:USifrs-full:GrossCarryingAmountMember2020-03-310001004315ifrs-full:TradeReceivablesMembercountry:GBifrs-full:AccumulatedImpairmentMember2021-03-310001004315ifrs-full:TradeReceivablesMemberifrs-full:AccumulatedImpairmentMembercountry:US2021-03-310001004315ifrs-full:TradeReceivablesMemberifrs-full:AccumulatedImpairmentMember2021-03-310001004315ifrs-full:TradeReceivablesMembercountry:GBifrs-full:AccumulatedImpairmentMember2020-03-310001004315ifrs-full:TradeReceivablesMemberifrs-full:AccumulatedImpairmentMembercountry:US2020-03-310001004315ifrs-full:TradeReceivablesMemberifrs-full:AccumulatedImpairmentMember2020-03-310001004315nggtf:RetailCustomerReceivablesMember2021-03-310001004315nggtf:RetailCustomerReceivablesMember2020-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:TradeReceivablesMember2020-04-012021-03-310001004315ifrs-full:TradeReceivablesMemberifrs-full:TopOfRangeMember2020-04-012021-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:TradeReceivablesMembernggtf:CessationOfCustomerTerminationsAndCollectionsMember2019-04-012020-03-310001004315ifrs-full:TradeReceivablesMemberifrs-full:TopOfRangeMembernggtf:CessationOfCustomerTerminationsAndCollectionsMember2019-04-012020-03-310001004315nggtf:ImpactOfCOVID19Memberifrs-full:TradeReceivablesMember2019-04-012020-03-310001004315nggtf:UnbilledRevenueMember2021-03-310001004315nggtf:UnbilledRevenueMember2020-03-310001004315ifrs-full:CurrentMembernggtf:RetailCustomerReceivablesMember2021-03-310001004315ifrs-full:CurrentMembernggtf:RetailCustomerReceivablesMember2020-03-310001004315ifrs-full:LaterThanOneMonthAndNotLaterThanTwoMonthsMembernggtf:RetailCustomerReceivablesMember2021-03-310001004315ifrs-full:LaterThanOneMonthAndNotLaterThanTwoMonthsMembernggtf:RetailCustomerReceivablesMember2020-03-310001004315ifrs-full:LaterThanTwoMonthsAndNotLaterThanThreeMonthsMembernggtf:RetailCustomerReceivablesMember2021-03-310001004315ifrs-full:LaterThanTwoMonthsAndNotLaterThanThreeMonthsMembernggtf:RetailCustomerReceivablesMember2020-03-310001004315ifrs-full:LaterThanThreeMonthsAndNotLaterThanSixMonthsMembernggtf:RetailCustomerReceivablesMember2021-03-310001004315ifrs-full:LaterThanThreeMonthsAndNotLaterThanSixMonthsMembernggtf:RetailCustomerReceivablesMember2020-03-310001004315ifrs-full:LaterThanSixMonthsAndNotLaterThanOneYearMembernggtf:RetailCustomerReceivablesMember2021-03-310001004315ifrs-full:LaterThanSixMonthsAndNotLaterThanOneYearMembernggtf:RetailCustomerReceivablesMember2020-03-310001004315ifrs-full:LaterThanOneYearMembernggtf:RetailCustomerReceivablesMember2021-03-310001004315ifrs-full:LaterThanOneYearMembernggtf:RetailCustomerReceivablesMember2020-03-310001004315ifrs-full:NotLaterThanOneYearMember2021-03-310001004315ifrs-full:NotLaterThanOneYearMember2020-03-310001004315ifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2021-03-310001004315ifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2020-03-310001004315ifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2021-03-310001004315ifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2020-03-310001004315ifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember2021-03-310001004315ifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember2020-03-310001004315ifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember2021-03-310001004315ifrs-full:LaterThanFourYearsAndNotLaterThanFiveYearsMember2020-03-310001004315nggtf:BorrowingsPaidByInstalmentsMemberifrs-full:LaterThanFiveYearsMember2021-03-310001004315nggtf:BorrowingsPaidByInstalmentsMemberifrs-full:LaterThanFiveYearsMember2020-03-310001004315nggtf:BorrowingsPaidByOtherThanInstalmentsMemberifrs-full:LaterThanFiveYearsMember2021-03-310001004315nggtf:BorrowingsPaidByOtherThanInstalmentsMemberifrs-full:LaterThanFiveYearsMember2020-03-310001004315ifrs-full:AtFairValueMember2021-03-310001004315ifrs-full:AtFairValueMember2020-03-310001004315ifrs-full:Level1OfFairValueHierarchyMemberifrs-full:AtFairValueMember2021-03-310001004315ifrs-full:Level1OfFairValueHierarchyMemberifrs-full:AtFairValueMember2020-03-310001004315ifrs-full:AtFairValueMemberifrs-full:Level2OfFairValueHierarchyMember2021-03-310001004315ifrs-full:AtFairValueMemberifrs-full:Level2OfFairValueHierarchyMember2020-03-310001004315nggtf:Borrowings1Member2021-03-310001004315nggtf:Borrowings1Member2020-03-310001004315nggtf:LondonInterbankOfferedRateGBPLIBORMember2021-03-310001004315nggtf:LondonInterbankOfferedRateUSDLIBORMember2021-03-310001004315ifrs-full:FinancialLiabilitiesAtFairValueThroughProfitOrLossCategoryMember2021-03-310001004315ifrs-full:FinancialLiabilitiesAtFairValueThroughProfitOrLossCategoryMember2020-03-310001004315ifrs-full:LaterThanOneYearAndNotLaterThanFiveYearsMember2021-03-310001004315ifrs-full:LaterThanOneYearAndNotLaterThanFiveYearsMember2020-03-310001004315ifrs-full:LaterThanFiveYearsMember2021-03-310001004315ifrs-full:LaterThanFiveYearsMember2020-03-310001004315nggtf:TheNationalGridYouPlanMember2021-03-310001004315nggtf:NationalGridUKRetirementPlanMember2021-03-31nggtf:plan0001004315ifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315nggtf:NationalGridUKPensionSchemeSectionAMember2019-03-310001004315nggtf:NationalGridUKPensionSchemeSectionBMember2019-03-310001004315nggtf:NationalGridElectricityGroupOfTheElectricitySupplyPensionSchemeMember2019-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:NationalGridUKPensionSchemeSectionAMember2019-03-310001004315nggtf:NationalGridUKPensionSchemeSectionBMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembernggtf:NationalGridUKPensionSchemeSectionAMember2020-12-310001004315ifrs-full:PensionDefinedBenefitPlansMembernggtf:NationalGridUKPensionSchemeSectionAMember2020-03-310001004315nggtf:NationalGridUKPensionSchemeSectionBMemberifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembernggtf:NationalGridUKPensionSchemeMember2020-04-012021-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembernggtf:NationalGridUKPensionSchemeMember2019-04-012020-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:TimePeriod2020To2022Membernggtf:NationalGridElectricityGroupOfTheElectricitySupplyPensionSchemeMember2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:TimePeriod2023Membernggtf:NationalGridElectricityGroupOfTheElectricitySupplyPensionSchemeMember2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:NationalGridElectricityGroupOfTheElectricitySupplyPensionSchemeMember2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:NationalGridUKPensionSchemeSectionAMember2021-03-310001004315nggtf:NationalGridUKPensionSchemeSectionBMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315country:GBnggtf:NationalGridUKPensionSchemeSectionAMember2021-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-04-012021-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-04-012020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembercountry:US2020-04-012021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembercountry:US2019-04-012020-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembercountry:US2019-03-31nggtf:year0001004315country:GBifrs-full:PensionDefinedBenefitPlansMember2020-04-012021-03-310001004315nggtf:ActiveMembersMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315nggtf:ActiveMembersMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315nggtf:ActiveMembersMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315nggtf:DeferredMembersMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315nggtf:DeferredMembersMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315nggtf:DeferredMembersMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:PensionerMembersMember2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:PensionerMembersMember2020-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:PensionerMembersMember2019-03-310001004315nggtf:ActiveMembersMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315nggtf:ActiveMembersMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315nggtf:ActiveMembersMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315nggtf:DeferredMembersMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315nggtf:DeferredMembersMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315nggtf:DeferredMembersMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembernggtf:PensionerMembersMembercountry:US2021-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembernggtf:PensionerMembersMembercountry:US2020-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembernggtf:PensionerMembersMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:ActiveMembersMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:ActiveMembersMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:ActiveMembersMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:PensionerMembersMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:PensionerMembersMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:PensionerMembersMembercountry:US2019-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMember2021-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMember2020-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMember2019-03-310001004315ifrs-full:WhollyUnfundedDefinedBenefitPlansMemberifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315ifrs-full:WhollyUnfundedDefinedBenefitPlansMemberifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315ifrs-full:WhollyUnfundedDefinedBenefitPlansMemberifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMember2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMember2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMember2019-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMember2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMember2020-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMember2019-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMembercountry:US2021-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMembercountry:US2020-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:WhollyOrPartlyFundedDefinedBenefitPlansMembercountry:US2019-03-310001004315ifrs-full:WhollyUnfundedDefinedBenefitPlansMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315ifrs-full:WhollyUnfundedDefinedBenefitPlansMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315ifrs-full:WhollyUnfundedDefinedBenefitPlansMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315ifrs-full:WhollyUnfundedDefinedBenefitPlansMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315ifrs-full:WhollyUnfundedDefinedBenefitPlansMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315ifrs-full:WhollyUnfundedDefinedBenefitPlansMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:WhollyUnfundedDefinedBenefitPlansMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:WhollyUnfundedDefinedBenefitPlansMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:WhollyUnfundedDefinedBenefitPlansMembercountry:US2019-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMember2019-04-012020-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMember2018-04-012019-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembercountry:US2018-04-012019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembercountry:US2018-04-012019-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMember2018-03-310001004315ifrs-full:PensionDefinedBenefitPlansMembercountry:US2018-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembercountry:US2018-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMember2020-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMember2019-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMember2018-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMember2020-04-012021-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMember2019-04-012020-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMember2018-04-012019-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMember2021-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2018-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2018-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:US2018-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-04-012021-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-04-012020-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2018-04-012019-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-04-012021-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-04-012020-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2018-04-012019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:US2020-04-012021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:US2019-04-012020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:US2018-04-012019-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315ifrs-full:PresentValueOfDefinedBenefitObligationMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PresentValueOfDefinedBenefitObligationMembercountry:US2021-03-310001004315ifrs-full:PlanAssetsMember2020-03-310001004315ifrs-full:PlanAssetsMember2019-03-310001004315ifrs-full:PlanAssetsMember2018-03-310001004315ifrs-full:PlanAssetsMember2020-04-012021-03-310001004315ifrs-full:PlanAssetsMember2019-04-012020-03-310001004315ifrs-full:PlanAssetsMember2018-04-012019-03-310001004315ifrs-full:PlanAssetsMember2021-03-310001004315ifrs-full:PlanAssetsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315ifrs-full:PlanAssetsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315ifrs-full:PlanAssetsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2018-03-310001004315ifrs-full:PlanAssetsMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315ifrs-full:PlanAssetsMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315ifrs-full:PlanAssetsMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2018-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PlanAssetsMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PlanAssetsMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PlanAssetsMembercountry:US2018-03-310001004315ifrs-full:PlanAssetsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-04-012021-03-310001004315ifrs-full:PlanAssetsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-04-012020-03-310001004315ifrs-full:PlanAssetsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2018-04-012019-03-310001004315ifrs-full:PlanAssetsMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-04-012021-03-310001004315ifrs-full:PlanAssetsMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-04-012020-03-310001004315ifrs-full:PlanAssetsMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2018-04-012019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PlanAssetsMembercountry:US2020-04-012021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PlanAssetsMembercountry:US2019-04-012020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PlanAssetsMembercountry:US2018-04-012019-03-310001004315ifrs-full:PlanAssetsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315ifrs-full:PlanAssetsMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:PlanAssetsMembercountry:US2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMember2021-03-310001004315ifrs-full:Level2And3OfFairValueHierarchyMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMember2020-03-310001004315ifrs-full:Level2And3OfFairValueHierarchyMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMember2019-03-310001004315ifrs-full:Level2And3OfFairValueHierarchyMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315nggtf:CorporateBondsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMember2021-03-310001004315nggtf:CorporateBondsMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315nggtf:CorporateBondsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315nggtf:CorporateBondsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMember2020-03-310001004315nggtf:CorporateBondsMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315nggtf:CorporateBondsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315nggtf:CorporateBondsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMember2019-03-310001004315nggtf:CorporateBondsMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315nggtf:CorporateBondsMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315nggtf:GovernmentSecuritiesMembercountry:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMember2021-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315nggtf:GovernmentSecuritiesMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315nggtf:GovernmentSecuritiesMembercountry:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMember2020-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315nggtf:GovernmentSecuritiesMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315nggtf:GovernmentSecuritiesMembercountry:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMember2019-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315nggtf:GovernmentSecuritiesMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2019-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2021-03-310001004315ifrs-full:Level2And3OfFairValueHierarchyMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2020-03-310001004315ifrs-full:Level2And3OfFairValueHierarchyMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2019-03-310001004315ifrs-full:Level2And3OfFairValueHierarchyMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315nggtf:CorporateBondsMemberifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2021-03-310001004315nggtf:CorporateBondsMemberifrs-full:Level2And3OfFairValueHierarchyMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315nggtf:CorporateBondsMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315nggtf:CorporateBondsMemberifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2020-03-310001004315nggtf:CorporateBondsMemberifrs-full:Level2And3OfFairValueHierarchyMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315nggtf:CorporateBondsMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315nggtf:CorporateBondsMemberifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2019-03-310001004315nggtf:CorporateBondsMemberifrs-full:Level2And3OfFairValueHierarchyMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315nggtf:CorporateBondsMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2021-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:Level2And3OfFairValueHierarchyMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2021-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2020-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:Level2And3OfFairValueHierarchyMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:PensionDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2019-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:Level2And3OfFairValueHierarchyMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315nggtf:GovernmentSecuritiesMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:CorporateBondsMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:CorporateBondsMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:CorporateBondsMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:CorporateBondsMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:CorporateBondsMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:CorporateBondsMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:CorporateBondsMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:CorporateBondsMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:CorporateBondsMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:GovernmentSecuritiesMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:GovernmentSecuritiesMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:GovernmentSecuritiesMembercountry:US2021-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:GovernmentSecuritiesMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:GovernmentSecuritiesMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:GovernmentSecuritiesMembercountry:US2020-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:GovernmentSecuritiesMemberifrs-full:Level1OfFairValueHierarchyMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:GovernmentSecuritiesMemberifrs-full:Level2And3OfFairValueHierarchyMembercountry:US2019-03-310001004315ifrs-full:PostemploymentMedicalDefinedBenefitPlansMembernggtf:GovernmentSecuritiesMembercountry:US2019-03-310001004315ifrs-full:OtherEnvironmentRelatedProvisionMember2019-03-310001004315ifrs-full:ProvisionForDecommissioningRestorationAndRehabilitationCostsMember2019-03-310001004315ifrs-full:RestructuringProvisionMember2019-03-310001004315ifrs-full:MiscellaneousOtherProvisionsMember2019-03-310001004315ifrs-full:ProvisionForDecommissioningRestorationAndRehabilitationCostsMember2019-04-012020-03-310001004315ifrs-full:RestructuringProvisionMember2019-04-012020-03-310001004315ifrs-full:MiscellaneousOtherProvisionsMember2019-04-012020-03-310001004315ifrs-full:OtherEnvironmentRelatedProvisionMember2020-03-310001004315ifrs-full:ProvisionForDecommissioningRestorationAndRehabilitationCostsMember2020-03-310001004315ifrs-full:RestructuringProvisionMember2020-03-310001004315ifrs-full:MiscellaneousOtherProvisionsMember2020-03-310001004315ifrs-full:OtherEnvironmentRelatedProvisionMember2020-04-012021-03-310001004315ifrs-full:ProvisionForDecommissioningRestorationAndRehabilitationCostsMember2020-04-012021-03-310001004315ifrs-full:RestructuringProvisionMember2020-04-012021-03-310001004315ifrs-full:MiscellaneousOtherProvisionsMember2020-04-012021-03-310001004315ifrs-full:OtherEnvironmentRelatedProvisionMember2021-03-310001004315ifrs-full:ProvisionForDecommissioningRestorationAndRehabilitationCostsMember2021-03-310001004315ifrs-full:RestructuringProvisionMember2021-03-310001004315ifrs-full:MiscellaneousOtherProvisionsMember2021-03-310001004315nggtf:ExceptionalItemsMemberifrs-full:OtherEnvironmentRelatedProvisionMember2019-04-012020-03-310001004315country:GBifrs-full:OtherEnvironmentRelatedProvisionMember2021-03-310001004315country:GBifrs-full:OtherEnvironmentRelatedProvisionMember2020-04-012021-03-310001004315country:GBifrs-full:OtherEnvironmentRelatedProvisionMember2020-03-310001004315ifrs-full:OtherEnvironmentRelatedProvisionMembercountry:US2020-04-012021-03-310001004315ifrs-full:OtherEnvironmentRelatedProvisionMembercountry:US2020-03-310001004315nggtf:ProvisionfordecommissioningAROMember2021-03-310001004315nggtf:ProvisionfordecommissioningAROMember2020-03-310001004315nggtf:ProvisionfordecommissioningdemolitionofgasholdersMember2021-03-310001004315nggtf:ProvisionfordecommissioningdemolitionofgasholdersMember2020-03-310001004315nggtf:LiabilityforClaimsandClaimsAdjustmentMember2021-03-310001004315nggtf:LiabilityforClaimsandClaimsAdjustmentMember2020-03-310001004315nggtf:BusinessSetupandRestructuringCostProvisionMember2021-03-310001004315nggtf:BusinessSetupandRestructuringCostProvisionMember2020-03-310001004315nggtf:OnerousLeaseCommitmentsAndRatesPayableOnSurplusPropertiesMember2021-03-310001004315nggtf:OnerousLeaseCommitmentsAndRatesPayableOnSurplusPropertiesMember2020-03-310001004315nggtf:EmissionsProvisionMember2021-03-310001004315nggtf:EmissionsProvisionMember2020-03-310001004315ifrs-full:ReserveOfExchangeDifferencesOnTranslationMembersrt:ScenarioPreviouslyReportedMember2018-03-310001004315srt:ScenarioPreviouslyReportedMemberifrs-full:ReserveOfCashFlowHedgesMember2018-03-310001004315srt:ScenarioPreviouslyReportedMemberifrs-full:ReserveOfGainsAndLossesOnRemeasuringAvailableforsaleFinancialAssetsMember2018-03-310001004315srt:ScenarioPreviouslyReportedMemberifrs-full:CapitalRedemptionReserveMember2018-03-310001004315srt:ScenarioPreviouslyReportedMemberifrs-full:MergerReserveMember2018-03-310001004315srt:ScenarioPreviouslyReportedMember2018-03-310001004315ifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMembernggtf:IFRS9Memberifrs-full:ReserveOfCashFlowHedgesMember2018-04-010001004315ifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMembernggtf:IFRS9Membernggtf:ReserveOfCostOfHedgingMember2018-04-010001004315ifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMembernggtf:IFRS9Memberifrs-full:ReserveOfGainsAndLossesOnRemeasuringAvailableforsaleFinancialAssetsMember2018-04-010001004315ifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMembernggtf:IFRS9Memberifrs-full:ReserveOfGainsAndLossesFromInvestmentsInEquityInstrumentsMember2018-04-010001004315ifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMembernggtf:IFRS9Membernggtf:ReserveOfGainsandLossesOnFinancialLiabilitiesMeasuredAtFairValueThroughOtherComprehensiveIncomeMember2018-04-010001004315ifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMemberifrs-full:ReserveOfChangeInFairValueOfFinancialLiabilityAttributableToChangeInCreditRiskOfLiabilityMembernggtf:IFRS9Member2018-04-010001004315ifrs-full:IncreaseDecreaseDueToChangesInAccountingPolicyRequiredByIFRSsMembernggtf:IFRS9Member2018-04-010001004315ifrs-full:ReserveOfExchangeDifferencesOnTranslationMember2018-04-010001004315ifrs-full:ReserveOfCashFlowHedgesMember2018-04-010001004315nggtf:ReserveOfCostOfHedgingMember2018-04-010001004315ifrs-full:ReserveOfGainsAndLossesFromInvestmentsInEquityInstrumentsMember2018-04-010001004315nggtf:ReserveOfGainsandLossesOnFinancialLiabilitiesMeasuredAtFairValueThroughOtherComprehensiveIncomeMember2018-04-010001004315ifrs-full:ReserveOfChangeInFairValueOfFinancialLiabilityAttributableToChangeInCreditRiskOfLiabilityMember2018-04-010001004315ifrs-full:CapitalRedemptionReserveMember2018-04-010001004315ifrs-full:MergerReserveMember2018-04-010001004315ifrs-full:ReserveOfExchangeDifferencesOnTranslationMember2018-04-012019-03-310001004315ifrs-full:ReserveOfCashFlowHedgesMember2018-04-012019-03-310001004315nggtf:ReserveOfCostOfHedgingMember2018-04-012019-03-310001004315nggtf:ReserveOfGainsandLossesOnFinancialLiabilitiesMeasuredAtFairValueThroughOtherComprehensiveIncomeMember2018-04-012019-03-310001004315ifrs-full:ReserveOfChangeInFairValueOfFinancialLiabilityAttributableToChangeInCreditRiskOfLiabilityMember2018-04-012019-03-310001004315ifrs-full:ReserveOfExchangeDifferencesOnTranslationMember2019-03-310001004315ifrs-full:ReserveOfCashFlowHedgesMember2019-03-310001004315nggtf:ReserveOfCostOfHedgingMember2019-03-310001004315srt:ScenarioPreviouslyReportedMemberifrs-full:ReserveOfGainsAndLossesOnRemeasuringAvailableforsaleFinancialAssetsMember2019-03-310001004315ifrs-full:ReserveOfGainsAndLossesFromInvestmentsInEquityInstrumentsMember2019-03-310001004315nggtf:ReserveOfGainsandLossesOnFinancialLiabilitiesMeasuredAtFairValueThroughOtherComprehensiveIncomeMember2019-03-310001004315ifrs-full:ReserveOfChangeInFairValueOfFinancialLiabilityAttributableToChangeInCreditRiskOfLiabilityMember2019-03-310001004315ifrs-full:CapitalRedemptionReserveMember2019-03-310001004315ifrs-full:MergerReserveMember2019-03-310001004315ifrs-full:ReserveOfExchangeDifferencesOnTranslationMember2019-04-012020-03-310001004315ifrs-full:ReserveOfCashFlowHedgesMember2019-04-012020-03-310001004315nggtf:ReserveOfCostOfHedgingMember2019-04-012020-03-310001004315ifrs-full:ReserveOfGainsAndLossesFromInvestmentsInEquityInstrumentsMember2019-04-012020-03-310001004315nggtf:ReserveOfGainsandLossesOnFinancialLiabilitiesMeasuredAtFairValueThroughOtherComprehensiveIncomeMember2019-04-012020-03-310001004315ifrs-full:ReserveOfChangeInFairValueOfFinancialLiabilityAttributableToChangeInCreditRiskOfLiabilityMember2019-04-012020-03-310001004315ifrs-full:ReserveOfExchangeDifferencesOnTranslationMember2020-03-310001004315ifrs-full:ReserveOfCashFlowHedgesMember2020-03-310001004315nggtf:ReserveOfCostOfHedgingMember2020-03-310001004315ifrs-full:ReserveOfGainsAndLossesFromInvestmentsInEquityInstrumentsMember2020-03-310001004315nggtf:ReserveOfGainsandLossesOnFinancialLiabilitiesMeasuredAtFairValueThroughOtherComprehensiveIncomeMember2020-03-310001004315ifrs-full:ReserveOfChangeInFairValueOfFinancialLiabilityAttributableToChangeInCreditRiskOfLiabilityMember2020-03-310001004315ifrs-full:CapitalRedemptionReserveMember2020-03-310001004315ifrs-full:MergerReserveMember2020-03-310001004315ifrs-full:ReserveOfExchangeDifferencesOnTranslationMember2020-04-012021-03-310001004315ifrs-full:ReserveOfCashFlowHedgesMember2020-04-012021-03-310001004315nggtf:ReserveOfCostOfHedgingMember2020-04-012021-03-310001004315ifrs-full:ReserveOfGainsAndLossesFromInvestmentsInEquityInstrumentsMember2020-04-012021-03-310001004315nggtf:ReserveOfGainsandLossesOnFinancialLiabilitiesMeasuredAtFairValueThroughOtherComprehensiveIncomeMember2020-04-012021-03-310001004315ifrs-full:ReserveOfChangeInFairValueOfFinancialLiabilityAttributableToChangeInCreditRiskOfLiabilityMember2020-04-012021-03-310001004315ifrs-full:ReserveOfExchangeDifferencesOnTranslationMember2021-03-310001004315ifrs-full:ReserveOfCashFlowHedgesMember2021-03-310001004315nggtf:ReserveOfCostOfHedgingMember2021-03-310001004315ifrs-full:ReserveOfGainsAndLossesFromInvestmentsInEquityInstrumentsMember2021-03-310001004315nggtf:ReserveOfGainsandLossesOnFinancialLiabilitiesMeasuredAtFairValueThroughOtherComprehensiveIncomeMember2021-03-310001004315ifrs-full:ReserveOfChangeInFairValueOfFinancialLiabilityAttributableToChangeInCreditRiskOfLiabilityMember2021-03-310001004315ifrs-full:CapitalRedemptionReserveMember2021-03-310001004315ifrs-full:MergerReserveMember2021-03-3100010043152019-04-010001004315nggtf:NoncurrentAssetsMember2021-03-310001004315nggtf:CurrentAssetsMember2021-03-310001004315nggtf:CurrentLiabilitiesMember2021-03-310001004315nggtf:BorrowingsMember2020-04-012021-03-310001004315nggtf:BorrowingsMember2019-04-012020-03-310001004315nggtf:BorrowingsMember2018-04-012019-03-310001004315nggtf:FinancingDerivativesMember2020-04-012021-03-310001004315nggtf:FinancingDerivativesMember2019-04-012020-03-310001004315nggtf:FinancingDerivativesMember2018-04-012019-03-310001004315nggtf:BorrowingsMember2018-03-310001004315nggtf:FinancingDerivativesMember2018-03-310001004315nggtf:BorrowingsMember2019-03-310001004315nggtf:FinancingDerivativesMember2019-03-310001004315nggtf:BorrowingsMember2020-03-310001004315nggtf:FinancingDerivativesMember2020-03-310001004315nggtf:BorrowingsMember2021-03-310001004315nggtf:FinancingDerivativesMember2021-03-310001004315nggtf:GuaranteeOfSubleaseForUSPropertyExpires2040Member2021-03-310001004315nggtf:GuaranteeOfSubleaseForUSPropertyExpires2040Member2020-03-310001004315nggtf:GuaranteesOfCertainObligationsOfGrainLNGImportTerminalExpireUpTo2025Member2021-03-310001004315nggtf:GuaranteesOfCertainObligationsOfGrainLNGImportTerminalExpireUpTo2025Member2020-03-310001004315nggtf:GuaranteesOfCertainObligationsForConstructionOfHVDCWestCoastLinkExpectedExpiry2059Member2021-03-310001004315nggtf:GuaranteesOfCertainObligationsForConstructionOfHVDCWestCoastLinkExpectedExpiry2059Member2020-03-310001004315nggtf:GuaranteesOfCertainObligationsOfNationalGridNorthSeaLinkLimitedVariousExpiryDatesMember2021-03-310001004315nggtf:GuaranteesOfCertainObligationsOfNationalGridNorthSeaLinkLimitedVariousExpiryDatesMember2020-03-310001004315nggtf:GuaranteesOfCertainObligationsOfStWilliamHomesLLPVariousExpiryDatesMember2021-03-310001004315nggtf:GuaranteesOfCertainObligationsOfStWilliamHomesLLPVariousExpiryDatesMember2020-03-310001004315nggtf:GuaranteesOfCertainObligationsOfNationalGridIFA2ExpectedExpiry2022Member2021-03-310001004315nggtf:GuaranteesOfCertainObligationsOfNationalGridIFA2ExpectedExpiry2022Member2020-03-310001004315nggtf:GuaranteesOfCertainObligationsofNationalGridVikingLinkLimitedexpectedexpiry2024Member2021-03-310001004315nggtf:GuaranteesOfCertainObligationsofNationalGridVikingLinkLimitedexpectedexpiry2024Member2020-03-310001004315nggtf:OtherGuaranteesAndLettersOfCreditVariousExpiryDatesMember2021-03-310001004315nggtf:OtherGuaranteesAndLettersOfCreditVariousExpiryDatesMember2020-03-310001004315nggtf:GuaranteestoBothJointVenturesandEPCContractorsRegardingTheConstructionofInterconnectorsMember2021-03-310001004315nggtf:GuaranteestoBothJointVenturesandEPCContractorsRegardingTheConstructionofInterconnectorsMember2020-03-310001004315nggtf:PensionPlanMember2020-04-012021-03-310001004315nggtf:PensionPlanMember2019-04-012020-03-310001004315nggtf:PensionPlanMember2018-04-012019-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMember2020-04-012021-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMember2019-04-012020-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMember2018-04-012019-03-310001004315ifrs-full:AssociatesMember2020-04-012021-03-310001004315ifrs-full:AssociatesMember2019-04-012020-03-310001004315ifrs-full:AssociatesMember2018-04-012019-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMember2021-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMember2020-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMember2019-03-310001004315ifrs-full:AssociatesMember2021-03-310001004315ifrs-full:AssociatesMember2020-03-310001004315ifrs-full:AssociatesMember2019-03-310001004315nggtf:StWilliamHomesLLPMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2020-04-012021-03-310001004315nggtf:StWilliamHomesLLPMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2019-04-012020-03-310001004315nggtf:StWilliamHomesLLPMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2018-04-012019-03-310001004315nggtf:EmeraldEnergyVentureLLCMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2020-04-012021-03-310001004315nggtf:EmeraldEnergyVentureLLCMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2019-04-012020-03-310001004315nggtf:EmeraldEnergyVentureLLCMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2018-04-012019-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMembernggtf:NGETSPTUpgradesLimitedMember2020-04-012021-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMembernggtf:NGETSPTUpgradesLimitedMember2019-04-012020-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMembernggtf:NGETSPTUpgradesLimitedMember2018-04-012019-03-310001004315nggtf:QuadgasHoldcoLimitedCadentMemberifrs-full:AssociatesMember2019-06-012019-06-300001004315nggtf:QuadgasHoldCoLimitedMemberifrs-full:AssociatesMember2019-04-012020-03-310001004315nggtf:QuadgasHoldCoLimitedMemberifrs-full:AssociatesMember2018-04-012019-03-310001004315ifrs-full:AssociatesMembernggtf:MillenniumPipelineCompanyLLCMember2020-04-012021-03-310001004315ifrs-full:AssociatesMembernggtf:MillenniumPipelineCompanyLLCMember2019-04-012020-03-310001004315ifrs-full:AssociatesMembernggtf:MillenniumPipelineCompanyLLCMember2018-04-012019-03-310001004315nggtf:StWilliamHomesLLPMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2021-03-310001004315nggtf:StWilliamHomesLLPMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2020-03-310001004315nggtf:StWilliamHomesLLPMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2019-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMembernggtf:QuadgasHoldCoLimitedMember2021-03-310001004315nggtf:NemoLinkLimitedMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2021-03-310001004315nggtf:QuadgasHoldCoLimitedMemberifrs-full:AssociatesMember2020-03-310001004315nggtf:QuadgasHoldCoLimitedMemberifrs-full:AssociatesMember2019-03-310001004315nggtf:NemoLinkLimitedMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2020-03-310001004315nggtf:NemoLinkLimitedMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2019-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMembernggtf:BritNedDevelopmentLimitedMember2020-04-012021-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMembernggtf:BritNedDevelopmentLimitedMember2019-04-012020-03-310001004315ifrs-full:JointVenturesWhereEntityIsVenturerMembernggtf:BritNedDevelopmentLimitedMember2018-04-012019-03-310001004315nggtf:NemoLinkLimitedMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2020-04-012021-03-310001004315nggtf:NemoLinkLimitedMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2019-04-012020-03-310001004315nggtf:NemoLinkLimitedMemberifrs-full:JointVenturesWhereEntityIsVenturerMember2018-04-012019-03-310001004315nggtf:AAAG7SovereignEntitiesMember2021-03-310001004315ifrs-full:LaterThanOneYearMembernggtf:AAAG7SovereignEntitiesMember2021-03-310001004315nggtf:AAAInvestmentVehiclesMember2021-03-310001004315ifrs-full:LaterThanOneYearMembernggtf:AAAInvestmentVehiclesMember2021-03-310001004315nggtf:AAAInstitutionsandNonG7SovereignEntitiesMember2021-03-310001004315ifrs-full:LaterThanOneYearMembernggtf:AAAInstitutionsandNonG7SovereignEntitiesMember2021-03-310001004315nggtf:AAPlusG7SovereignEntitiesMember2021-03-310001004315ifrs-full:LaterThanOneYearMembernggtf:AAPlusG7SovereignEntitiesMember2021-03-310001004315ifrs-full:BottomOfRangeMembernggtf:AAInstitutionsMember2021-03-310001004315ifrs-full:TopOfRangeMembernggtf:AAInstitutionsMember2021-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:LaterThanOneYearMembernggtf:AAInstitutionsMember2021-03-310001004315ifrs-full:LaterThanOneYearMemberifrs-full:TopOfRangeMembernggtf:AAInstitutionsMember2021-03-310001004315ifrs-full:BottomOfRangeMembernggtf:AInstitutionsMember2021-03-310001004315ifrs-full:TopOfRangeMembernggtf:AInstitutionsMember2021-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:LaterThanOneYearMembernggtf:AInstitutionsMember2021-03-310001004315ifrs-full:LaterThanOneYearMemberifrs-full:TopOfRangeMembernggtf:AInstitutionsMember2021-03-310001004315nggtf:BankAccountandBankOverdraftMemberifrs-full:CreditRiskMember2021-03-310001004315nggtf:BankAccountandBankOverdraftMemberifrs-full:CreditRiskMember2020-03-310001004315ifrs-full:CreditRiskMembernggtf:FinancingDerivativesMember2021-03-310001004315ifrs-full:CreditRiskMembernggtf:CommodityDerivativesMember2021-03-310001004315ifrs-full:CreditRiskMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:CreditRiskMember2021-03-310001004315ifrs-full:CreditRiskMembernggtf:CommodityDerivativesMember2021-03-310001004315ifrs-full:CreditRiskMembernggtf:FinancingDerivativesMember2020-03-310001004315ifrs-full:CreditRiskMembernggtf:CommodityDerivativesMember2020-03-310001004315ifrs-full:CreditRiskMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:CreditRiskMember2020-03-310001004315ifrs-full:CreditRiskMembernggtf:CommodityDerivativesMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMember2021-03-310001004315ifrs-full:LiquidityRiskMember2021-03-310001004315nggtf:FinancingDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMember2021-03-310001004315nggtf:FinancingDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2021-03-310001004315nggtf:FinancingDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2021-03-310001004315nggtf:FinancingDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMember2021-03-310001004315nggtf:FinancingDerivativesReceiptsMemberifrs-full:LiquidityRiskMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:FinancingDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMembernggtf:FinancingDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:FinancingDerivativesPaymentsMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMembernggtf:FinancingDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:FinancingDerivativesPaymentsMember2021-03-310001004315nggtf:CommodityDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMember2021-03-310001004315nggtf:CommodityDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2021-03-310001004315nggtf:CommodityDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2021-03-310001004315nggtf:CommodityDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMember2021-03-310001004315nggtf:CommodityDerivativesReceiptsMemberifrs-full:LiquidityRiskMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:CommodityDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMembernggtf:CommodityDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMembernggtf:CommodityDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMembernggtf:CommodityDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:FinancingDerivativesReceiptsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMembernggtf:FinancingDerivativesReceiptsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMembernggtf:FinancingDerivativesReceiptsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMembernggtf:FinancingDerivativesReceiptsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:FinancingDerivativesReceiptsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:FinancingDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMembernggtf:FinancingDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMembernggtf:FinancingDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMembernggtf:FinancingDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:FinancingDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:CommodityDerivativesReceiptsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesReceiptsMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesReceiptsMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesReceiptsMemberifrs-full:LaterThanThreeYearsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesReceiptsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:CommodityDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesPaymentsMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesPaymentsMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMembernggtf:CommodityDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesPaymentsMember2021-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMember2020-03-310001004315ifrs-full:LiquidityRiskMember2020-03-310001004315nggtf:FinancingDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMember2020-03-310001004315nggtf:FinancingDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2020-03-310001004315nggtf:FinancingDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2020-03-310001004315nggtf:FinancingDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMember2020-03-310001004315nggtf:FinancingDerivativesReceiptsMemberifrs-full:LiquidityRiskMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:FinancingDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMembernggtf:FinancingDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:FinancingDerivativesPaymentsMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMembernggtf:FinancingDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:FinancingDerivativesPaymentsMember2020-03-310001004315nggtf:CommodityDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMember2020-03-310001004315nggtf:CommodityDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2020-03-310001004315nggtf:CommodityDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2020-03-310001004315nggtf:CommodityDerivativesReceiptsMemberifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMember2020-03-310001004315nggtf:CommodityDerivativesReceiptsMemberifrs-full:LiquidityRiskMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:CommodityDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMembernggtf:CommodityDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMembernggtf:CommodityDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMembernggtf:CommodityDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:FinancingDerivativesReceiptsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMembernggtf:FinancingDerivativesReceiptsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMembernggtf:FinancingDerivativesReceiptsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMembernggtf:FinancingDerivativesReceiptsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:FinancingDerivativesReceiptsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:FinancingDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMembernggtf:FinancingDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMembernggtf:FinancingDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMembernggtf:FinancingDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:FinancingDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:CommodityDerivativesReceiptsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesReceiptsMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesReceiptsMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesReceiptsMemberifrs-full:LaterThanThreeYearsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesReceiptsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:NotLaterThanOneYearMembernggtf:CommodityDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesPaymentsMemberifrs-full:LaterThanOneYearAndNotLaterThanTwoYearsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesPaymentsMemberifrs-full:LaterThanTwoYearsAndNotLaterThanThreeYearsMember2020-03-310001004315ifrs-full:LiquidityRiskMemberifrs-full:LaterThanThreeYearsMembernggtf:CommodityDerivativesPaymentsMember2020-03-310001004315ifrs-full:LiquidityRiskMembernggtf:CommodityDerivativesPaymentsMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:CashandCashEquivalentsMembercurrency:GBP2021-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:CashandCashEquivalentsMember2021-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:CashandCashEquivalentsMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:CashandCashEquivalentsMembernggtf:OtherCurrencyMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:CashandCashEquivalentsMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:CashandCashEquivalentsMembercurrency:GBP2020-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:CashandCashEquivalentsMember2020-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:CashandCashEquivalentsMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:CashandCashEquivalentsMembernggtf:OtherCurrencyMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:CashandCashEquivalentsMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:FinancialInvestmentsMembercurrency:GBP2021-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:FinancialInvestmentsMember2021-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:FinancialInvestmentsMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:FinancialInvestmentsMembernggtf:OtherCurrencyMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:FinancialInvestmentsMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:FinancialInvestmentsMembercurrency:GBP2020-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:FinancialInvestmentsMember2020-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:FinancialInvestmentsMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:FinancialInvestmentsMembernggtf:OtherCurrencyMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:FinancialInvestmentsMember2020-03-310001004315ifrs-full:CurrencyRiskMembercurrency:GBPnggtf:Borrowings1Member2021-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:Borrowings1Member2021-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:Borrowings1Member2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:OtherCurrencyMembernggtf:Borrowings1Member2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:Borrowings1Member2021-03-310001004315ifrs-full:CurrencyRiskMembercurrency:GBPnggtf:Borrowings1Member2020-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:Borrowings1Member2020-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:Borrowings1Member2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:OtherCurrencyMembernggtf:Borrowings1Member2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:Borrowings1Member2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:PrederivativePositionMembercurrency:GBP2021-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:PrederivativePositionMember2021-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:PrederivativePositionMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:PrederivativePositionMembernggtf:OtherCurrencyMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:PrederivativePositionMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:PrederivativePositionMembercurrency:GBP2020-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:PrederivativePositionMember2020-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:PrederivativePositionMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:PrederivativePositionMembernggtf:OtherCurrencyMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:PrederivativePositionMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:CurrencyRiskMembercurrency:GBP2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:CurrencyRiskMembercurrency:EUR2021-03-310001004315nggtf:FinancingDerivativesMembercurrency:USDifrs-full:CurrencyRiskMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:CurrencyRiskMembernggtf:OtherCurrencyMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:CurrencyRiskMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:CurrencyRiskMembercurrency:GBP2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:CurrencyRiskMembercurrency:EUR2020-03-310001004315nggtf:FinancingDerivativesMembercurrency:USDifrs-full:CurrencyRiskMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:CurrencyRiskMembernggtf:OtherCurrencyMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:CurrencyRiskMember2020-03-310001004315ifrs-full:CurrencyRiskMembercurrency:GBP2021-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EUR2021-03-310001004315currency:USDifrs-full:CurrencyRiskMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:OtherCurrencyMember2021-03-310001004315ifrs-full:CurrencyRiskMember2021-03-310001004315ifrs-full:CurrencyRiskMembercurrency:GBP2020-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EUR2020-03-310001004315currency:USDifrs-full:CurrencyRiskMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:OtherCurrencyMember2020-03-310001004315ifrs-full:CurrencyRiskMember2020-03-310001004315ifrs-full:CurrencyRiskMembercurrency:GBPnggtf:TradeandOtherReceivablesMember2021-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:TradeandOtherReceivablesMember2021-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:TradeandOtherReceivablesMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:TradeandOtherReceivablesMembernggtf:OtherCurrencyMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:TradeandOtherReceivablesMember2021-03-310001004315ifrs-full:CurrencyRiskMembercurrency:GBPnggtf:TradeandOtherReceivablesMember2020-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:TradeandOtherReceivablesMember2020-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:TradeandOtherReceivablesMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:TradeandOtherReceivablesMembernggtf:OtherCurrencyMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:TradeandOtherReceivablesMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:TradeAndOtherPayablesMembercurrency:GBP2021-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:TradeAndOtherPayablesMember2021-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:TradeAndOtherPayablesMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:TradeAndOtherPayablesMembernggtf:OtherCurrencyMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:TradeAndOtherPayablesMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:TradeAndOtherPayablesMembercurrency:GBP2020-03-310001004315ifrs-full:CurrencyRiskMembercurrency:EURnggtf:TradeAndOtherPayablesMember2020-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:TradeAndOtherPayablesMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:TradeAndOtherPayablesMembernggtf:OtherCurrencyMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:TradeAndOtherPayablesMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:NoncurrentLiabilitiesMembercurrency:GBP2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:NoncurrentLiabilitiesMembercurrency:EUR2021-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:NoncurrentLiabilitiesMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:NoncurrentLiabilitiesMembernggtf:OtherCurrencyMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:NoncurrentLiabilitiesMember2021-03-310001004315ifrs-full:CurrencyRiskMembernggtf:NoncurrentLiabilitiesMembercurrency:GBP2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:NoncurrentLiabilitiesMembercurrency:EUR2020-03-310001004315currency:USDifrs-full:CurrencyRiskMembernggtf:NoncurrentLiabilitiesMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:NoncurrentLiabilitiesMembernggtf:OtherCurrencyMember2020-03-310001004315ifrs-full:CurrencyRiskMembernggtf:NoncurrentLiabilitiesMember2020-03-310001004315ifrs-full:FixedInterestRateMembernggtf:CashandCashEquivalentsMemberifrs-full:InterestRateRiskMember2021-03-310001004315ifrs-full:FloatingInterestRateMembernggtf:CashandCashEquivalentsMemberifrs-full:InterestRateRiskMember2021-03-310001004315nggtf:CashandCashEquivalentsMemberifrs-full:InterestRateRiskMembernggtf:InflationLinkedInterestRateMember2021-03-310001004315nggtf:OtherInterestRateMembernggtf:CashandCashEquivalentsMemberifrs-full:InterestRateRiskMember2021-03-310001004315nggtf:CashandCashEquivalentsMemberifrs-full:InterestRateRiskMember2021-03-310001004315ifrs-full:FixedInterestRateMembernggtf:CashandCashEquivalentsMemberifrs-full:InterestRateRiskMember2020-03-310001004315ifrs-full:FloatingInterestRateMembernggtf:CashandCashEquivalentsMemberifrs-full:InterestRateRiskMember2020-03-310001004315nggtf:CashandCashEquivalentsMemberifrs-full:InterestRateRiskMembernggtf:InflationLinkedInterestRateMember2020-03-310001004315nggtf:OtherInterestRateMembernggtf:CashandCashEquivalentsMemberifrs-full:InterestRateRiskMember2020-03-310001004315nggtf:CashandCashEquivalentsMemberifrs-full:InterestRateRiskMember2020-03-310001004315ifrs-full:FixedInterestRateMemberifrs-full:InterestRateRiskMembernggtf:FinancialInvestmentsMember2021-03-310001004315ifrs-full:FloatingInterestRateMemberifrs-full:InterestRateRiskMembernggtf:FinancialInvestmentsMember2021-03-310001004315ifrs-full:InterestRateRiskMembernggtf:FinancialInvestmentsMembernggtf:InflationLinkedInterestRateMember2021-03-310001004315nggtf:OtherInterestRateMemberifrs-full:InterestRateRiskMembernggtf:FinancialInvestmentsMember2021-03-310001004315ifrs-full:InterestRateRiskMembernggtf:FinancialInvestmentsMember2021-03-310001004315ifrs-full:FixedInterestRateMemberifrs-full:InterestRateRiskMembernggtf:FinancialInvestmentsMember2020-03-310001004315ifrs-full:FloatingInterestRateMemberifrs-full:InterestRateRiskMembernggtf:FinancialInvestmentsMember2020-03-310001004315ifrs-full:InterestRateRiskMembernggtf:FinancialInvestmentsMembernggtf:InflationLinkedInterestRateMember2020-03-310001004315nggtf:OtherInterestRateMemberifrs-full:InterestRateRiskMembernggtf:FinancialInvestmentsMember2020-03-310001004315ifrs-full:InterestRateRiskMembernggtf:FinancialInvestmentsMember2020-03-310001004315ifrs-full:FixedInterestRateMemberifrs-full:InterestRateRiskMembernggtf:Borrowings1Member2021-03-310001004315ifrs-full:FloatingInterestRateMemberifrs-full:InterestRateRiskMembernggtf:Borrowings1Member2021-03-310001004315ifrs-full:InterestRateRiskMembernggtf:InflationLinkedInterestRateMembernggtf:Borrowings1Member2021-03-310001004315nggtf:OtherInterestRateMemberifrs-full:InterestRateRiskMembernggtf:Borrowings1Member2021-03-310001004315ifrs-full:InterestRateRiskMembernggtf:Borrowings1Member2021-03-310001004315ifrs-full:FixedInterestRateMemberifrs-full:InterestRateRiskMembernggtf:Borrowings1Member2020-03-310001004315ifrs-full:FloatingInterestRateMemberifrs-full:InterestRateRiskMembernggtf:Borrowings1Member2020-03-310001004315ifrs-full:InterestRateRiskMembernggtf:InflationLinkedInterestRateMembernggtf:Borrowings1Member2020-03-310001004315nggtf:OtherInterestRateMemberifrs-full:InterestRateRiskMembernggtf:Borrowings1Member2020-03-310001004315ifrs-full:InterestRateRiskMembernggtf:Borrowings1Member2020-03-310001004315ifrs-full:FixedInterestRateMembernggtf:PrederivativePositionMemberifrs-full:InterestRateRiskMember2021-03-310001004315ifrs-full:FloatingInterestRateMembernggtf:PrederivativePositionMemberifrs-full:InterestRateRiskMember2021-03-310001004315nggtf:PrederivativePositionMemberifrs-full:InterestRateRiskMembernggtf:InflationLinkedInterestRateMember2021-03-310001004315nggtf:PrederivativePositionMembernggtf:OtherInterestRateMemberifrs-full:InterestRateRiskMember2021-03-310001004315nggtf:PrederivativePositionMemberifrs-full:InterestRateRiskMember2021-03-310001004315ifrs-full:FixedInterestRateMembernggtf:PrederivativePositionMemberifrs-full:InterestRateRiskMember2020-03-310001004315ifrs-full:FloatingInterestRateMembernggtf:PrederivativePositionMemberifrs-full:InterestRateRiskMember2020-03-310001004315nggtf:PrederivativePositionMemberifrs-full:InterestRateRiskMembernggtf:InflationLinkedInterestRateMember2020-03-310001004315nggtf:PrederivativePositionMembernggtf:OtherInterestRateMemberifrs-full:InterestRateRiskMember2020-03-310001004315nggtf:PrederivativePositionMemberifrs-full:InterestRateRiskMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:FixedInterestRateMemberifrs-full:InterestRateRiskMember2021-03-310001004315ifrs-full:FloatingInterestRateMembernggtf:FinancingDerivativesMemberifrs-full:InterestRateRiskMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:InterestRateRiskMembernggtf:InflationLinkedInterestRateMember2021-03-310001004315nggtf:FinancingDerivativesMembernggtf:OtherInterestRateMemberifrs-full:InterestRateRiskMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:InterestRateRiskMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:FixedInterestRateMemberifrs-full:InterestRateRiskMember2020-03-310001004315ifrs-full:FloatingInterestRateMembernggtf:FinancingDerivativesMemberifrs-full:InterestRateRiskMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:InterestRateRiskMembernggtf:InflationLinkedInterestRateMember2020-03-310001004315nggtf:FinancingDerivativesMembernggtf:OtherInterestRateMemberifrs-full:InterestRateRiskMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:InterestRateRiskMember2020-03-310001004315ifrs-full:FixedInterestRateMemberifrs-full:InterestRateRiskMember2021-03-310001004315ifrs-full:FloatingInterestRateMemberifrs-full:InterestRateRiskMember2021-03-310001004315ifrs-full:InterestRateRiskMembernggtf:InflationLinkedInterestRateMember2021-03-310001004315nggtf:OtherInterestRateMemberifrs-full:InterestRateRiskMember2021-03-310001004315ifrs-full:InterestRateRiskMember2021-03-310001004315ifrs-full:FixedInterestRateMemberifrs-full:InterestRateRiskMember2020-03-310001004315ifrs-full:FloatingInterestRateMemberifrs-full:InterestRateRiskMember2020-03-310001004315ifrs-full:InterestRateRiskMembernggtf:InflationLinkedInterestRateMember2020-03-310001004315nggtf:OtherInterestRateMemberifrs-full:InterestRateRiskMember2020-03-310001004315ifrs-full:InterestRateRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:MarketRiskMember2020-04-012021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMember2020-04-012021-03-310001004315ifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2020-04-012021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMember2020-04-012021-03-310001004315nggtf:ReserveOfCostOfHedgingMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2021-03-310001004315ifrs-full:CashFlowHedgesMembernggtf:ReserveOfCostOfHedgingMemberifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMembernggtf:ReserveOfCostOfHedgingMember2021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMembernggtf:ReserveOfCostOfHedgingMember2021-03-310001004315nggtf:CurrentDerivativeFinancialAssetsMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2021-03-310001004315ifrs-full:CashFlowHedgesMembernggtf:CurrentDerivativeFinancialAssetsMemberifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMembernggtf:CurrentDerivativeFinancialAssetsMember2021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMembernggtf:CurrentDerivativeFinancialAssetsMember2021-03-310001004315ifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMembernggtf:NonCurrentDerivativeFinancialAssetsMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:MarketRiskMembernggtf:NonCurrentDerivativeFinancialAssetsMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMembernggtf:NonCurrentDerivativeFinancialAssetsMember2021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMembernggtf:NonCurrentDerivativeFinancialAssetsMember2021-03-310001004315nggtf:CurrentDerivativeFinancialLiabilitiesMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2021-03-310001004315ifrs-full:CashFlowHedgesMembernggtf:CurrentDerivativeFinancialLiabilitiesMemberifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMembernggtf:CurrentDerivativeFinancialLiabilitiesMember2021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMembernggtf:CurrentDerivativeFinancialLiabilitiesMember2021-03-310001004315nggtf:NonCurrentDerivativeFinancialLiabilitiesMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2021-03-310001004315ifrs-full:CashFlowHedgesMembernggtf:NonCurrentDerivativeFinancialLiabilitiesMemberifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMembernggtf:NonCurrentDerivativeFinancialLiabilitiesMember2021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMembernggtf:NonCurrentDerivativeFinancialLiabilitiesMember2021-03-31iso4217:GBPiso4217:USD0001004315ifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2021-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:CashFlowHedgesMemberifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:TopOfRangeMemberifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMemberifrs-full:TopOfRangeMember2021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMemberifrs-full:BottomOfRangeMember2021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMemberifrs-full:TopOfRangeMember2021-03-31iso4217:GBPiso4217:EUR0001004315ifrs-full:BottomOfRangeMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2021-03-310001004315ifrs-full:TopOfRangeMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2021-03-31iso4217:EURiso4217:USD0001004315ifrs-full:BottomOfRangeMembercurrency:GBPifrs-full:MarketRiskMembernggtf:LondonInterbankOfferedRateLIBOR1Memberifrs-full:FairValueHedgesMember2021-03-310001004315ifrs-full:TopOfRangeMembercurrency:GBPifrs-full:MarketRiskMembernggtf:LondonInterbankOfferedRateLIBOR1Memberifrs-full:FairValueHedgesMember2021-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:CashFlowHedgesMembercurrency:GBPifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:TopOfRangeMembercurrency:GBPifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:BottomOfRangeMembercurrency:USDifrs-full:MarketRiskMembernggtf:LondonInterbankOfferedRateLIBOR1Memberifrs-full:FairValueHedgesMember2021-03-310001004315currency:USDifrs-full:TopOfRangeMemberifrs-full:MarketRiskMembernggtf:LondonInterbankOfferedRateLIBOR1Memberifrs-full:FairValueHedgesMember2021-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:CashFlowHedgesMembercurrency:USDifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMembercurrency:USDifrs-full:TopOfRangeMemberifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:MarketRiskMember2019-04-012020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMember2019-04-012020-03-310001004315ifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2019-04-012020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMember2019-04-012020-03-310001004315nggtf:ReserveOfCostOfHedgingMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:CashFlowHedgesMembernggtf:ReserveOfCostOfHedgingMemberifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMembernggtf:ReserveOfCostOfHedgingMember2020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMembernggtf:ReserveOfCostOfHedgingMember2020-03-310001004315nggtf:CurrentDerivativeFinancialAssetsMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:CashFlowHedgesMembernggtf:CurrentDerivativeFinancialAssetsMemberifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMembernggtf:CurrentDerivativeFinancialAssetsMember2020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMembernggtf:CurrentDerivativeFinancialAssetsMember2020-03-310001004315ifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMembernggtf:NonCurrentDerivativeFinancialAssetsMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:MarketRiskMembernggtf:NonCurrentDerivativeFinancialAssetsMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMembernggtf:NonCurrentDerivativeFinancialAssetsMember2020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMembernggtf:NonCurrentDerivativeFinancialAssetsMember2020-03-310001004315nggtf:CurrentDerivativeFinancialLiabilitiesMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:CashFlowHedgesMembernggtf:CurrentDerivativeFinancialLiabilitiesMemberifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMembernggtf:CurrentDerivativeFinancialLiabilitiesMember2020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMembernggtf:CurrentDerivativeFinancialLiabilitiesMember2020-03-310001004315nggtf:NonCurrentDerivativeFinancialLiabilitiesMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:CashFlowHedgesMembernggtf:NonCurrentDerivativeFinancialLiabilitiesMemberifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMembernggtf:NonCurrentDerivativeFinancialLiabilitiesMember2020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMembernggtf:NonCurrentDerivativeFinancialLiabilitiesMember2020-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:CashFlowHedgesMemberifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:TopOfRangeMemberifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMemberifrs-full:TopOfRangeMember2020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMemberifrs-full:BottomOfRangeMember2020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMemberifrs-full:TopOfRangeMember2020-03-310001004315ifrs-full:TopOfRangeMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMember2020-03-310001004315ifrs-full:BottomOfRangeMembercurrency:GBPifrs-full:MarketRiskMembernggtf:LondonInterbankOfferedRateLIBOR1Memberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:TopOfRangeMembercurrency:GBPifrs-full:MarketRiskMembernggtf:LondonInterbankOfferedRateLIBOR1Memberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:CashFlowHedgesMembercurrency:GBPifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:TopOfRangeMembercurrency:GBPifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:BottomOfRangeMembercurrency:USDifrs-full:MarketRiskMembernggtf:LondonInterbankOfferedRateLIBOR1Memberifrs-full:FairValueHedgesMember2020-03-310001004315currency:USDifrs-full:TopOfRangeMemberifrs-full:MarketRiskMembernggtf:LondonInterbankOfferedRateLIBOR1Memberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:BottomOfRangeMemberifrs-full:CashFlowHedgesMembercurrency:USDifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMembercurrency:USDifrs-full:TopOfRangeMemberifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMembernggtf:Borrowings1Member2021-03-310001004315ifrs-full:MarketRiskMembernggtf:CurrentLiabilitiesMemberifrs-full:FairValueHedgesMembernggtf:Borrowings1Member2021-03-310001004315nggtf:NoncurrentLiabilitiesMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMembernggtf:Borrowings1Member2021-03-310001004315nggtf:ImpactedByIFRS9InterestRateBenchmarkReformAmendmentsMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2021-03-310001004315ifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMembernggtf:Borrowings1Member2020-03-310001004315ifrs-full:MarketRiskMembernggtf:CurrentLiabilitiesMemberifrs-full:FairValueHedgesMembernggtf:Borrowings1Member2020-03-310001004315nggtf:NoncurrentLiabilitiesMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMembernggtf:Borrowings1Member2020-03-310001004315nggtf:ImpactedByIFRS9InterestRateBenchmarkReformAmendmentsMemberifrs-full:MarketRiskMemberifrs-full:FairValueHedgesMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMembernggtf:ImpactedByIFRS9InterestRateBenchmarkReformAmendmentsMemberifrs-full:MarketRiskMember2021-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMemberifrs-full:CurrencyRiskMember2020-03-310001004315ifrs-full:CashFlowHedgesMembernggtf:ImpactedByIFRS9InterestRateBenchmarkReformAmendmentsMemberifrs-full:MarketRiskMember2020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMemberifrs-full:CurrencyRiskMember2021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMemberifrs-full:CurrencyRiskMember2020-04-012021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMemberifrs-full:CurrencyRiskMembernggtf:ImpactedByIFRS9InterestRateBenchmarkReformAmendmentsMember2021-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMemberifrs-full:CurrencyRiskMember2020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMemberifrs-full:CurrencyRiskMember2019-04-012020-03-310001004315ifrs-full:HedgesOfNetInvestmentInForeignOperationsMemberifrs-full:CurrencyRiskMembernggtf:ImpactedByIFRS9InterestRateBenchmarkReformAmendmentsMember2020-03-310001004315nggtf:InvestmentsheldatFVTPLMemberifrs-full:Level1OfFairValueHierarchyMember2021-03-310001004315nggtf:InvestmentsheldatFVTPLMemberifrs-full:Level2OfFairValueHierarchyMember2021-03-310001004315nggtf:InvestmentsheldatFVTPLMemberifrs-full:Level3OfFairValueHierarchyMember2021-03-310001004315nggtf:InvestmentsheldatFVTPLMember2021-03-310001004315nggtf:InvestmentsheldatFVTPLMemberifrs-full:Level1OfFairValueHierarchyMember2020-03-310001004315nggtf:InvestmentsheldatFVTPLMemberifrs-full:Level2OfFairValueHierarchyMember2020-03-310001004315nggtf:InvestmentsheldatFVTPLMemberifrs-full:Level3OfFairValueHierarchyMember2020-03-310001004315nggtf:InvestmentsheldatFVTPLMember2020-03-310001004315nggtf:InvestmentsheldatFVTOCIMemberifrs-full:Level1OfFairValueHierarchyMember2021-03-310001004315nggtf:InvestmentsheldatFVTOCIMemberifrs-full:Level2OfFairValueHierarchyMember2021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:InvestmentsheldatFVTOCIMember2021-03-310001004315nggtf:InvestmentsheldatFVTOCIMember2021-03-310001004315nggtf:InvestmentsheldatFVTOCIMemberifrs-full:Level1OfFairValueHierarchyMember2020-03-310001004315nggtf:InvestmentsheldatFVTOCIMemberifrs-full:Level2OfFairValueHierarchyMember2020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:InvestmentsheldatFVTOCIMember2020-03-310001004315nggtf:InvestmentsheldatFVTOCIMember2020-03-310001004315ifrs-full:Level1OfFairValueHierarchyMembernggtf:JointVenturesandAssociatesMember2021-03-310001004315ifrs-full:Level2OfFairValueHierarchyMembernggtf:JointVenturesandAssociatesMember2021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:JointVenturesandAssociatesMember2021-03-310001004315nggtf:JointVenturesandAssociatesMember2021-03-310001004315ifrs-full:Level1OfFairValueHierarchyMembernggtf:JointVenturesandAssociatesMember2020-03-310001004315ifrs-full:Level2OfFairValueHierarchyMembernggtf:JointVenturesandAssociatesMember2020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:JointVenturesandAssociatesMember2020-03-310001004315nggtf:JointVenturesandAssociatesMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level1OfFairValueHierarchyMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level2OfFairValueHierarchyMember2021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:FinancingDerivativesMember2021-03-310001004315nggtf:FinancingDerivativesMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level1OfFairValueHierarchyMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level2OfFairValueHierarchyMember2020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:FinancingDerivativesMember2020-03-310001004315nggtf:FinancingDerivativesMember2020-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:Level1OfFairValueHierarchyMember2021-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:Level2OfFairValueHierarchyMember2021-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:Level3OfFairValueHierarchyMember2021-03-310001004315nggtf:CommodityDerivativesMember2021-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:Level1OfFairValueHierarchyMember2020-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:Level2OfFairValueHierarchyMember2020-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:Level3OfFairValueHierarchyMember2020-03-310001004315nggtf:CommodityDerivativesMember2020-03-310001004315ifrs-full:Level1OfFairValueHierarchyMember2021-03-310001004315ifrs-full:Level2OfFairValueHierarchyMember2021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMember2021-03-310001004315ifrs-full:Level1OfFairValueHierarchyMember2020-03-310001004315ifrs-full:Level2OfFairValueHierarchyMember2020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level1OfFairValueHierarchyMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level2OfFairValueHierarchyMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level3OfFairValueHierarchyMember2021-03-310001004315nggtf:FinancingDerivativesMember2021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level1OfFairValueHierarchyMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level2OfFairValueHierarchyMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level3OfFairValueHierarchyMember2020-03-310001004315nggtf:FinancingDerivativesMember2020-03-310001004315ifrs-full:Level1OfFairValueHierarchyMembernggtf:CommodityDerivativesMember2021-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:Level2OfFairValueHierarchyMember2021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:CommodityDerivativesMember2021-03-310001004315nggtf:CommodityDerivativesMember2021-03-310001004315ifrs-full:Level1OfFairValueHierarchyMembernggtf:CommodityDerivativesMember2020-03-310001004315nggtf:CommodityDerivativesMemberifrs-full:Level2OfFairValueHierarchyMember2020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:CommodityDerivativesMember2020-03-310001004315nggtf:CommodityDerivativesMember2020-03-310001004315ifrs-full:Level1OfFairValueHierarchyMembernggtf:LiabilitiesHeldatFairValueMember2021-03-310001004315ifrs-full:Level2OfFairValueHierarchyMembernggtf:LiabilitiesHeldatFairValueMember2021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:LiabilitiesHeldatFairValueMember2021-03-310001004315nggtf:LiabilitiesHeldatFairValueMember2021-03-310001004315ifrs-full:Level1OfFairValueHierarchyMembernggtf:LiabilitiesHeldatFairValueMember2020-03-310001004315ifrs-full:Level2OfFairValueHierarchyMembernggtf:LiabilitiesHeldatFairValueMember2020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:LiabilitiesHeldatFairValueMember2020-03-310001004315nggtf:LiabilitiesHeldatFairValueMember2020-03-310001004315ifrs-full:ContingentConsiderationMemberifrs-full:Level1OfFairValueHierarchyMember2021-03-310001004315ifrs-full:ContingentConsiderationMemberifrs-full:Level2OfFairValueHierarchyMember2021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMemberifrs-full:ContingentConsiderationMember2021-03-310001004315ifrs-full:ContingentConsiderationMember2021-03-310001004315ifrs-full:ContingentConsiderationMemberifrs-full:Level1OfFairValueHierarchyMember2020-03-310001004315ifrs-full:ContingentConsiderationMemberifrs-full:Level2OfFairValueHierarchyMember2020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMemberifrs-full:ContingentConsiderationMember2020-03-310001004315ifrs-full:ContingentConsiderationMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level3OfFairValueHierarchyMember2020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level3OfFairValueHierarchyMember2019-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:CommodityDerivativesMember2020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:CommodityDerivativesMember2019-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:InvestmentsheldatFVTPLMember2020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:InvestmentsheldatFVTPLMember2019-03-310001004315ifrs-full:Level3OfFairValueHierarchyMember2019-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level3OfFairValueHierarchyMember2020-04-012021-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level3OfFairValueHierarchyMember2019-04-012020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:CommodityDerivativesMember2020-04-012021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:CommodityDerivativesMember2019-04-012020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:InvestmentsheldatFVTPLMember2020-04-012021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:InvestmentsheldatFVTPLMember2019-04-012020-03-310001004315ifrs-full:Level3OfFairValueHierarchyMember2020-04-012021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMember2019-04-012020-03-310001004315nggtf:FinancingDerivativesMemberifrs-full:Level3OfFairValueHierarchyMember2021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:CommodityDerivativesMember2021-03-310001004315ifrs-full:Level3OfFairValueHierarchyMembernggtf:InvestmentsheldatFVTPLMember2021-03-310001004315nggtf:CommodityPriceRiskTenPercentIncreaseMembernggtf:CommodityDerivativesMember2021-03-310001004315nggtf:CommodityPriceRiskTenPercentIncreaseMembernggtf:CommodityDerivativesMember2020-03-310001004315nggtf:CommodityDerivativesMembernggtf:CommodityPriceRiskTenPercentDecreaseMember2021-03-310001004315nggtf:CommodityDerivativesMembernggtf:CommodityPriceRiskTenPercentDecreaseMember2020-03-310001004315nggtf:CommodityDerivativesMembernggtf:MarketRiskMarketAreaPriceTenPercentIncreaseMember2021-03-310001004315nggtf:CommodityDerivativesMembernggtf:MarketRiskMarketAreaPriceTenPercentIncreaseMember2020-03-310001004315nggtf:CommodityDerivativesMembernggtf:MarketRiskMarketAreaPriceTenPercentDecreaseMember2021-03-310001004315nggtf:CommodityDerivativesMembernggtf:MarketRiskMarketAreaPriceTenPercentDecreaseMember2020-03-310001004315nggtf:FinancingDerivativesMembernggtf:MarketRiskLimitedPriceInflationMarketCurveTwentyBasisPointsIncreaseMember2021-03-310001004315nggtf:FinancingDerivativesMembernggtf:MarketRiskLimitedPriceInflationMarketCurveTwentyBasisPointsIncreaseMember2020-03-310001004315nggtf:FinancingDerivativesMembernggtf:MarketRiskLimitedPriceInflationMarketCurveTwentyBasisPointsDecreaseMember2021-03-310001004315nggtf:FinancingDerivativesMembernggtf:MarketRiskLimitedPriceInflationMarketCurveTwentyBasisPointsDecreaseMember2020-03-310001004315nggtf:OtherFinancialInstrumentsMembernggtf:DiscountRateRisk50BasisPointsIncreaseMember2021-03-310001004315nggtf:OtherFinancialInstrumentsMembernggtf:DiscountRateRisk50BasisPointsIncreaseMember2020-03-310001004315nggtf:DiscountRateRisk50BasisPointsDecreaseMembernggtf:OtherFinancialInstrumentsMember2021-03-310001004315nggtf:DiscountRateRisk50BasisPointsDecreaseMembernggtf:OtherFinancialInstrumentsMember2020-03-310001004315nggtf:BilateralCommittedCreditFacilitiesMember2021-03-310001004315nggtf:BilateralCommittedCreditFacilitiesMember2020-03-310001004315nggtf:CreditFacilitiesFromSyndicatesOfBanksMember2021-03-310001004315nggtf:CreditFacilitiesFromSyndicatesOfBanksMember2020-03-310001004315nggtf:FacilitiesHeldAsBackupToCommercialPaperAndSimilarBorrowingsMember2021-03-310001004315nggtf:FacilitiesHeldAsBackupToCommercialPaperAndSimilarBorrowingsMember2020-03-310001004315nggtf:FacilitiesAvailableAsBackupToSpecificUSBorrowingsMember2021-03-310001004315nggtf:FacilitiesAvailableAsBackupToSpecificUSBorrowingsMember2020-03-310001004315nggtf:BilateralCommittedCreditFacilitiesMemberifrs-full:LaterThanThreeYearsAndNotLaterThanFourYearsMember2024-05-310001004315nggtf:A2019ExportCreditAgreementsMember2021-03-310001004315nggtf:A2019ExportCreditAgreementsMember2020-03-310001004315nggtf:ExportCreditAgencyMember2021-03-310001004315nggtf:ExportCreditAgencyMember2020-03-31nggtf:numberOfLoan0001004315nggtf:FacilitiesToFinanceTheAcquisitionMember2020-03-310001004315nggtf:FacilitiesToBackupAcquiredDebtMember2020-03-310001004315nggtf:FacilitiesCancelledAsWaiversObtainedMember2021-03-310001004315nggtf:WarwickTechnologyParkManagementCompanyNo2LimitedMember2020-04-012021-03-310001004315nggtf:NewEnglandHydroFinanceCompanyInc.Member2020-04-012021-03-310001004315nggtf:NewEnglandHydroTransmissionCorporationMember2020-04-012021-03-310001004315nggtf:NewEnglandHydroTransmissionElectricCompanyInc.Member2020-04-012021-03-310001004315nggtf:VermontGreenLineDevcoLLCMember2020-04-012021-03-310001004315nggtf:JointRadioCompanyLimitedMember2020-04-012021-03-310001004315nggtf:NationalPlacesLLPMember2020-04-012021-03-310001004315nggtf:NGETSPTUpgradesLimitedMember2020-04-012021-03-310001004315nggtf:StWilliamHomesLLPMember2020-04-012021-03-310001004315nggtf:BightWindHoldingsLLCMember2020-04-012021-03-310001004315nggtf:CleanEnergyGenerationLLCMember2020-04-012021-03-310001004315nggtf:IslandParkEnergyCenterLLCMember2020-04-012021-03-310001004315nggtf:IslanderEastPipelineCompanyLLCMember2020-04-012021-03-310001004315nggtf:LIEnergyStorageSystemLLCMember2020-04-012021-03-310001004315nggtf:LISolarGenerationLLCMember2020-04-012021-03-310001004315nggtf:IFA2SASMember2020-04-012021-03-310001004315nggtf:CleanLineEnergyPartnersLLCMember2020-04-012021-03-310001004315nggtf:ConnecticutYankeeAtomicPowerCompanyMember2020-04-012021-03-310001004315nggtf:DirectGlobalPowerInc.Member2020-04-012021-03-310001004315nggtf:EnergyImpactFundLPMember2020-04-012021-03-310001004315nggtf:KHBVentureLLCMember2020-04-012021-03-310001004315nggtf:MaineYankeeAtomicPowerCompanyMember2020-04-012021-03-310001004315nggtf:NewYorkTranscoLLCMember2020-04-012021-03-310001004315nggtf:NysearchRMLDLLCMember2020-04-012021-03-310001004315nggtf:TheHiveIVLLCMember2020-04-012021-03-310001004315nggtf:YankeeAtomicElectricCompanyMember2020-04-012021-03-310001004315nggtf:CoresoSAMember2020-04-012021-03-310001004315nggtf:EnergisplcMember2020-04-012021-03-310001004315nggtf:COrdinarySharesMembernggtf:BritNedDevelopmentLimitedMembernggtf:NationalGridInterconnectorHoldingsLimitedMember2021-03-31iso4217:EURxbrli:shares0001004315nggtf:COrdinarySharesMember2021-03-310001004315nggtf:AOrdinarySharesMembernggtf:BritNedDevelopmentLimitedMembernggtf:NationalGridInterconnectorHoldingsLimitedMember2021-03-310001004315nggtf:AOrdinarySharesMember2021-03-310001004315nggtf:NationalGridElectricityTransmissionplcMembernggtf:AOrdinarySharesMembernggtf:NGETSPTUpgradesLimitedMember2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:ActuarialAssumptionOfDiscountRatesMember2020-04-012021-03-310001004315country:GBifrs-full:ActuarialAssumptionOfDiscountRatesMember2020-04-012021-03-310001004315country:GBifrs-full:ActuarialAssumptionOfDiscountRatesMember2021-03-310001004315country:GBifrs-full:ActuarialAssumptionOfDiscountRatesMember2019-04-012020-03-310001004315country:GBifrs-full:ActuarialAssumptionOfDiscountRatesMember2020-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:ActuarialAssumptionOfDiscountRatesMembercountry:US2020-04-012021-03-310001004315ifrs-full:ActuarialAssumptionOfDiscountRatesMembercountry:US2020-04-012021-03-310001004315ifrs-full:ActuarialAssumptionOfDiscountRatesMembercountry:US2021-03-310001004315ifrs-full:ActuarialAssumptionOfDiscountRatesMembercountry:US2019-04-012020-03-310001004315ifrs-full:ActuarialAssumptionOfDiscountRatesMembercountry:US2020-03-310001004315ifrs-full:ActuarialAssumptionOfExpectedRatesOfInflationMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-04-012021-03-310001004315ifrs-full:ActuarialAssumptionOfExpectedRatesOfInflationMembercountry:GB2020-04-012021-03-310001004315ifrs-full:ActuarialAssumptionOfExpectedRatesOfInflationMembercountry:GB2021-03-310001004315ifrs-full:ActuarialAssumptionOfExpectedRatesOfInflationMembercountry:GB2019-04-012020-03-310001004315ifrs-full:ActuarialAssumptionOfExpectedRatesOfInflationMembercountry:GB2020-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMemberifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMember2020-04-012021-03-310001004315country:GBifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMember2020-04-012021-03-310001004315country:GBifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMember2021-03-310001004315country:GBifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMember2019-04-012020-03-310001004315country:GBifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMember2020-03-310001004315ifrs-full:PensionDefinedBenefitPlansMemberifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMembercountry:US2020-04-012021-03-310001004315ifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMembercountry:US2020-04-012021-03-310001004315ifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMembercountry:US2021-03-310001004315ifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMembercountry:US2019-04-012020-03-310001004315ifrs-full:ActuarialAssumptionOfExpectedRatesOfSalaryIncreasesMembercountry:US2020-03-310001004315ifrs-full:ActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-04-012021-03-310001004315ifrs-full:ActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:GB2020-04-012021-03-310001004315ifrs-full:ActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:GB2021-03-310001004315ifrs-full:ActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:GB2019-04-012020-03-310001004315ifrs-full:ActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:GB2020-03-310001004315ifrs-full:ActuarialAssumptionOfLifeExpectancyAfterRetirementMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-04-012021-03-310001004315ifrs-full:ActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:US2020-04-012021-03-310001004315ifrs-full:ActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:US2021-03-310001004315ifrs-full:ActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:US2019-04-012020-03-310001004315ifrs-full:ActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:US2020-03-310001004315nggtf:USHealthcareCostMemberifrs-full:PensionDefinedBenefitPlansMembercountry:US2020-04-012021-03-310001004315nggtf:USHealthcareCostMembercountry:US2020-04-012021-03-310001004315nggtf:USHealthcareCostMembercountry:US2021-03-310001004315nggtf:USHealthcareCostMembercountry:US2019-04-012020-03-310001004315nggtf:USHealthcareCostMembercountry:US2020-03-310001004315nggtf:EstimatedFutureCashFlowsChangeMemberifrs-full:OtherEnvironmentRelatedProvisionMember2020-04-012021-03-310001004315nggtf:EstimatedFutureCashFlowsChangeMemberifrs-full:OtherEnvironmentRelatedProvisionMember2021-03-310001004315nggtf:EstimatedFutureCashFlowsChangeMemberifrs-full:OtherEnvironmentRelatedProvisionMember2019-04-012020-03-310001004315nggtf:EstimatedFutureCashFlowsChangeMemberifrs-full:OtherEnvironmentRelatedProvisionMember2020-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:ValueOfBuyinPoliciesOffsetToActuarialAssumptionOfDiscountRateMember2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:ValueOfBuyinPoliciesOffsetToActuarialAssumptionOfDiscountRateMember2020-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:ValueOfBuyinPoliciesOffsetToActuarialAssumptionOfExpectedRatesOfInflationMember2021-03-310001004315country:GBifrs-full:PensionDefinedBenefitPlansMembernggtf:ValueOfBuyinPoliciesOffsetToActuarialAssumptionOfExpectedRatesOfInflationMember2020-03-310001004315nggtf:ValueOfBuyinPoliciesAndLongevitySwapOffsetToActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2021-03-310001004315nggtf:ValueOfBuyinPoliciesAndLongevitySwapOffsetToActuarialAssumptionOfLifeExpectancyAfterRetirementMembercountry:GBifrs-full:PensionDefinedBenefitPlansMember2020-03-310001004315country:GBifrs-full:OtherPriceRiskMember2020-04-012021-03-310001004315country:GBifrs-full:OtherPriceRiskMember2021-03-310001004315country:GBifrs-full:OtherPriceRiskMember2019-04-012020-03-310001004315country:GBifrs-full:OtherPriceRiskMember2020-03-310001004315country:GBifrs-full:InterestRateRiskMember2020-04-012021-03-310001004315country:GBifrs-full:InterestRateRiskMember2021-03-310001004315country:GBifrs-full:InterestRateRiskMember2019-04-012020-03-310001004315country:GBifrs-full:InterestRateRiskMember2020-03-310001004315ifrs-full:InterestRateRiskMembercountry:US2020-04-012021-03-310001004315ifrs-full:InterestRateRiskMembercountry:US2021-03-310001004315ifrs-full:InterestRateRiskMembercountry:US2019-04-012020-03-310001004315ifrs-full:InterestRateRiskMembercountry:US2020-03-310001004315ifrs-full:CurrencyRiskMember2020-04-012021-03-310001004315ifrs-full:CurrencyRiskMember2019-04-012020-03-310001004315ifrs-full:CommodityPriceRiskMember2020-04-012021-03-310001004315ifrs-full:CommodityPriceRiskMember2021-03-310001004315ifrs-full:CommodityPriceRiskMember2019-04-012020-03-310001004315ifrs-full:CommodityPriceRiskMember2020-03-310001004315nggtf:DerivativeFinancialInstrumentsMembernggtf:FairValueChangeMember2020-04-012021-03-310001004315nggtf:DerivativeFinancialInstrumentsMembernggtf:FairValueChangeMember2021-03-310001004315nggtf:DerivativeFinancialInstrumentsMembernggtf:FairValueChangeMember2019-04-012020-03-310001004315nggtf:DerivativeFinancialInstrumentsMembernggtf:FairValueChangeMember2020-03-310001004315nggtf:CommodityContractDerivativeLiabilitiesMembernggtf:FairValueChangeMember2020-04-012021-03-310001004315nggtf:CommodityContractDerivativeLiabilitiesMembernggtf:FairValueChangeMember2021-03-310001004315nggtf:CommodityContractDerivativeLiabilitiesMembernggtf:FairValueChangeMember2019-04-012020-03-310001004315nggtf:CommodityContractDerivativeLiabilitiesMembernggtf:FairValueChangeMember2020-03-310001004315nggtf:SubsidiaryGuarantorNationalGridGasplcMember2007-10-292007-10-290001004315nggtf:IssuerOfNotesBritishTranscoFinanceInc.Member2007-10-292007-10-290001004315nggtf:IssuerOfNotesBritishTranscoFinanceInc.Member2020-04-012021-03-310001004315nggtf:OthersubsidiariesMember2021-03-310001004315nggtf:ParentGuarantorAndNiagaraMohawkPowerCorporationMember2021-03-310001004315nggtf:ParentGuarantorAndNiagaraMohawkPowerCorporationMember2020-04-012021-03-310001004315nggtf:OthersubsidiariesMember2020-04-012021-03-310001004315nggtf:IFRS16Member2019-03-310001004315nggtf:IFRS16Membernggtf:ScenarioAdjustment1Member2019-04-010001004315nggtf:IFRS9Membernggtf:ScenarioAdjustment1Member2018-04-010001004315nggtf:IFRS15Membernggtf:ScenarioAdjustment1Member2018-04-010001004315ifrs-full:RetainedEarningsMembernggtf:FinancialAssetsAtFairValueThroughProfitOrLossAndThroughOtherComprehensiveIncomeCategoryMember2019-03-310001004315ifrs-full:FinancialAssetsAvailableforsaleCategoryMembernggtf:MoneyMarketFundsAndFundInvestmentsInEquitiesAndBondsMember2018-04-010001004315nggtf:MoneyMarketFundsAndFundInvestmentsInEquitiesAndBondsMember2018-04-010001004315ifrs-full:FinancialAssetsAtFairValueThroughProfitOrLossCategoryMembernggtf:MoneyMarketFundsAndFundInvestmentsInEquitiesAndBondsMember2018-04-010001004315ifrs-full:FinancialAssetsAvailableforsaleCategoryMembernggtf:CashSurrenderValueOfLifeInsurancePoliciesAndInvestmentsInDebtSecuritiesMember2018-04-010001004315nggtf:CashSurrenderValueOfLifeInsurancePoliciesAndInvestmentsInDebtSecuritiesMember2018-04-010001004315ifrs-full:FinancialAssetsAtFairValueThroughOtherComprehensiveIncomeCategoryMembernggtf:CashSurrenderValueOfLifeInsurancePoliciesAndInvestmentsInDebtSecuritiesMember2018-04-010001004315ifrs-full:FinancialAssetsAvailableforsaleCategoryMemberifrs-full:EquityInvestmentsMember2018-04-010001004315ifrs-full:EquityInvestmentsMember2018-04-010001004315ifrs-full:FinancialAssetsAtFairValueThroughOtherComprehensiveIncomeCategoryMemberifrs-full:EquityInvestmentsMember2018-04-010001004315ifrs-full:LoansAndReceivablesCategoryMembernggtf:LoansToJointVenturesAndAssociatesAndRestrictedBalancesMember2018-04-010001004315nggtf:LoansToJointVenturesAndAssociatesAndRestrictedBalancesMember2018-04-010001004315nggtf:LoansToJointVenturesAndAssociatesAndRestrictedBalancesMemberifrs-full:FinancialAssetsAtAmortisedCostCategoryMember2018-04-010001004315ifrs-full:FinancialLiabilitiesAtAmortisedCostCategoryMembernggtf:BorrowingsMember