what sort of cake are you thinking of? christmas style, pudding or cake, victoria sponge, bakewell tart, rock cake, I did try a date and walnut loaf on her a few months back, was good in theory but I did something wrong as it is now a doorstop!! (shouldn't forget my favourite cinnamon scones either!!
pre heat oven to 190c cream low fat spread and sugar together, beat in eggs one at a time and add 1TBS of flour with the second egg. fold in rest of the flour and baking powder together with the bananas. spread the mixture evenly in a lined and greased 15x25cm shallow tin and sprinkle walnuts over the top. bake for 20-25 mins leave mixture in the tin after it has cooked but cut into 16 or however many slices you want. leave the tin on a wire rack to cool and then turn out the slices when it has all cooled down.
calories for each slice (16) is about 100 and the carb rating is 10g per slice.