Cut the jam-filled sponge roll into even rounds and arrange a full layer over the bottom of a large trifle bowl.
Lightly drizzle the sponge with ginger ale to soften it and add flavour. You only need enough to moisten the sponge rather than soak it completely.
Pour over a generous layer of thick custard and spread gently to the edges.
Scatter a handful of fresh raspberries or sliced strawberries over the custard.
Whip the cream until soft peaks form, then spoon or pipe a layer over the berries.
Repeat the layers of sponge, ginger ale, custard, berries and cream until the bowl is full, finishing with a layer of whipped cream.
Decorate with extra fresh berries and grated or chopped chocolate, if desired.
Refrigerate for at least 7-8 hours, or overnight, to allow the layers to soften and the flavours to come together. I think it is best the next day.