Thats strange, just a UK sponge mix right?
This is what I do,
I only use "I can't believe its not butter" for some reason it makes the nicest sponge, even better than butter, its not so greasy.
I always cream the marg and sugar together first, and until the mixture looks a lighter yellow, so its creamed together well.
Then I add the eggs, with a pinch of salt and a little bit of milk (really small amount like half a teaspoon, it keeps the cake a bit moist) mix that in, just until its mixed in. Then I add the self raising flour in two halves, half in first and ALWAYS SIFT THE FLOUR, mix just until its combined, then the other half and mix in, in a folding action, this is to keep the air in the cake.
I hope this info will help! Good luck and keep trying!