Bring the raspberries, blackberries and blueberries to the boil in a saucepan with the sugar and a little water. Simmer for a few minutes until the fruits burst. Set aside.
Slice the sponge cake into thin pieces and place these in a bowl or in four individual soup plates. Spoon over the warm fruits and some of their syrup. Trickle over the custard and, if you wish, dust with icing sugar.