Place the butter in a medium pan with the maple syrup and melt over a gentle heat. Stir in the raisins. Remove from the heat and add the bananas, stirring well. Add the oats and stir well until all the oats have been coated.
Spoon the mixture into a 28 x 18 cm (11 x 7 inch) nonstick Swiss roll tin and level the surface using a potato masher for ease. Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 10 minutes until the top is just beginning to turn a pale golden. The mixture will still seem somewhat soft.
Allow to cool for 10 minutes in the tin before cutting into 12 squares. Remove from the tin and allow to cool completely.