Pre-heat the oven to 180/gas4. Butter a 900g loaf tin and line bottom and sides with non-stick baking parchment.
Peel and mash bananas. Put the sugar into a mixing bowl and cream together until light and fluffy. Stir in bananas and sultanas. Sift together the flour, baking powder and bicarbonate of soda. Add the flour mix and the beaten egg, each about a third at a time, to the banana mix, beating well between each addition. Finaly stir in the oil.
Pur into loaf tin and bake for 40-45 mins until well risen and golden brown and skewer inserted in the middle comes out clen. Leave in tin for 10 mins and then turn out on to a wire rack to cool completely.
When cool make frosting by creaming everything together. This makes enough to fill and decorate top.