Set oven to 170 C / Gas 3. Grease and lightly flour a 900g (2 lb) loaf tin or two 450g (1 lb) tins.
Cream together the margarine and sugar. Add beaten eggs, mashed bananas and mix thoroughly.
Sift together flour and baking powder in a separate bowl and add to the butter mixture.
Finally dissolve bicarbonate of soda in a small amount of warm water and stir in gently.
Pour the mixture into the prepared baking tin(s).
Bake in preheated oven for approximately 80 to 90 minutes until a skewer inserted into the centre comes out clean (if using 2 smaller tins, check the cakes 10 to 15 minutes earlier). Remove from the oven and turn out immediately onto cooling rack. Allow to cool before slicing and enjoying.