Line the bottom of a trifle bowl with sponge fingers and drizzle 3 tablespoons of sherry over them. Leave to stand for an hour so that the fingers can soak up the sherry.
Prepare the custard with the milk according to the instructions on the packet and leave to cool.
Soak the cut fruit in the rest of the sherry, add sugar if desired and spread over the sponge fingers.
Pour the cold custard over the fruit.
Finally, beat the cream with the vanilla and top the trifle with it. Decorate with the reserved strawberries. Chill until serving.