Combine egg yolks, sugar, rind and juice in top of double saucepan, stir over simmering water until custard mixture is slightly thickened.
Pour into large basin, cool to room temperature.
Beat egg whites until soft peaks form, add extra sugar, beat until dissolved, fold into custard mixture in two batches.
Beat cream until soft peaks form, fold gently into custard mixture.
Pour into 20cm x 10cm (8in x 4in) loaf tin, cover with aluminium foil, freeze overnight.