To make custard, put milk in a pan and bring to the boil. Whisk the sugar and egg yolks together in a large bowl. Pour hot milk in a thin, steady stream over the sugar and egg yolk mixture, whisking well. Return mixture to pan and heat until mixture starts to thicken, stirring continuously (do not boil). Leave to cool.
Line base and sides of a 1.4 litre bowl with Swiss roll slices.
Put lemon juice in a separate bowl and sprinkle gelatine over. Put over pan of hot water and heat till gelatine melts. Stir into custard.
Chill custard till it starts to thicken.
Whip cream until it forms soft peaks. Fold in custard and lemon zest.
Spoon into bowl lined with Swiss roll and leave until set (at least 4 to 5 hours). Just before serving turn out and decorate with fruit.