Cream the margarine and sugar together. Beat the eggs and beat them into the creamed mixture. Sift the flour and baking powder together and gradually add to the creamed mixture, along with sufficient milk to give a soft consistency.
Divide the sponge between two bowls. Dissolve the cocoa powder in the warm water and stir this into one of the bowls of sponge mix. Spoon the sponge mixtures alternately into the prepared tin, and blend the different colours with a skewer, but not too much - you want a marbled effect.
Bake in the preheated oven for about 30-35 minutes or until springy to the touch.
For chocolate sauce, stir a little cocoa powder into the milk before you add the custard powder and sugar.