Line the loaf tin with baking paper. Mash 3 of the bananas and put in a mixing bowl; slice the remaining banana lengthways and set aside.
Add the rest of the ingredients to the bowl and mix into a lumpy batter, ensuring there are no patches of dry flour left but trying not to overmix. Spoon the mixture into the lined tin, then sit the sliced banana on top.
Put the tin in the oven and turn the temperature to 160°C fan/gas 4. Bake for 1 hour, or until a skewer inserted in the centre of the banana bread comes out clean.
Leave to cool in the tin on a wire rack for 10-15 minutes, then remove the bread from the tin and serve warm or at room temperature.