Beat together the sugars and margarine in a mixing bowl until light and fluffy.
Add one of the ripe bananas and mix together.
Add the eggs one at a time and mix thoroughly between each egg.
Finally add the flour and California walnuts and fold together.
Pour into a 2lb loaf tin.
Cut the remaining banana longways and sit on top of the of the cake before baking in a pre heated oven 170C for 30-40 minutes or until the top of the cake is firm to the touch and it springs back when you press down.
Leave to cool before serving.
