i have a recipe for blue peter easter cake from years ago which uses a jar of mayo instead of butter or marg if that's any good.
10 oz. sr flour
8 oz. sugar
1 and a half tsps. baking powder
7 oz. jar mayo
4 tblspns. cocoa
8 fl. oz. hot water
1 tsp. vanilla essence
mix flour, sugar, baking powder and mayo until 'crummy'. dissolve cocoa and vanilla essence in hot water, add to flour, etc., and stir well. pour into a 7" lined tine and bake for 1 hour at gas mk. 4.
i used to cover it in chocolate icing and put mini eggs on top, but splitting it and filling with cherrries and cream sounds much better.