Mix the sugar and marg together until combined. Add the egg and 2 and a half of the bananas, squish bananas in hands to mash slightly when adding. Mix again until well combined. Add unsifted flour and bi-carb soda, mix well until combined.
Transfer to a baking paper-lined loaf tin. Slice remaining banana and arrange on top of mixture in pan. Cook for 45 to 60 minutes at 200 degrees C.
Take out when ready and let cool for 5 minutes before serving.
This recipe works well with freezing. Just cut the slices and wrap in cling wrap before putting in the freezer. The slices should keep for about 3 weeks.