Divide custard into two medium bowls, add melted dark chocolate melts into one bowl and whisk well. Add melted milk chocolate melts into other custard bowl, whisk well and set aside. Using an electric mixer, whip sweetened condensed milk and thickened cream into soft peaks.
Arrange half of sponge cake over the base of a 3 litre (12 cup) capacity serving dish. Spoon 3/4 of the prepared dark chocolate custard over the sponge base; top with 1/2 of the set jelly, 1/2 whipped cream mixture and 1/3 of the berries. Repeat, using sponge cake, all the milk chocolate custard, jelly, whipped cream mixture and 1/3 of berries.
Finish off with remaining dark chocolate custard and decorate with remaining berries. Dust with icing sugar if desired