Preheat oven to 180 degrees C. Grease and flour one 23x33cm tin, or two 18x8cm loaf tins.
Combine banana and sour cream. Set aside. In a large bowl, cream together the margarine and sugar until smooth. Beat in the eggs one at a time, then stir in the vanilla and banana mixture. Combine the flour, bicarb soda, baking powder and salt; stir into the banana mixture. Spread the batter evenly into the prepared tin or tins.
Bake for 50 minutes in the preheated oven,or until a toothpick inserted into the centre of the bread comes out clean.