Haven't tried the the Marks and Spencer one but this is a basic sponge recipe:
8 oz self raising flour
8 oz butter (I also use vitalite/pure and can't taste the difference but some people would disagree)
8 oz caster sugar
4 eggs
Splash of vanilla essence (optional but I prefer it with it in)
Cream together butter and sugar until pale and fluffy
Beat eggs together in separate bowl
Fold the eggs and flour into the creamed butter and sugar mixture. This needs to be done slowly, the eggs have to be added at the same time as the flour so they don't curdle and the flour should be sifted in.
Add the vanilla if using.
Divide between 2 greased and lined 8 inch tins and bake at 180 degrees in a preheated oven. Take out when baked. Should take about 20 minutes. You know it's done when the top of the cakes spring back when lightly pressed.
Take out of cake tins and leave to cool on wire rack.
Fill with strawberry or raspberry jam and either whipped cream or buttercream (to make buttercream beat together butter and icing sugar, no quantities sorry, I do it by taste.)
Dust top with icing sugar and eat.
You can change the quantities easily. I.e. 6 oz of all dry ingredients and 3 eggs, 4 oz of all dry ingredients and 2 eggs.
Only cake I can bake with no recipe.