Choc sponge is just sponge cake with cocoa. A really simple method to make any sponge is to weigh the eggs, say, an egg per person. So if you are a family of 4, weigh 4 eggs. Then you simply need the same weight for all other ingredients, margarine, caster sugar and self raising flour.
As you are making choc sponge, substitute one ounce flour for one ounce of cocoa.
Start with the marg and sugar, beat them till pale yellow and fluffy; add the eggs one at a time, beating each one in. Beat really well till really pale yellow and fluffy (5+ mins).
Fold in the sieved flour / cocoa. Bake in a sandwich tin or individual ramekins until a knife comes out clean (20 mins-ish)
Can you make custard? Add to the custard powder a few teaspoons of cocoa. Stir with a slug of cold milk and the sugar until it goes smooth then add to your hot milk.