Preheat the oven to 190 C / Gas 5. Butter the inside of a 20cm square baking dish.
In a bowl, mix together the oats, maple syrup, baking powder, salt and cinnamon.
In another bowl mix milk, eggs, and the vanilla.
Cut the bananas in 5mm pieces and place them on the bottom of the baking dish in a single layer along with half of the mixed berries. Cover the fruit with the oat mixture. Pour the milk mixture over the oats. Place the remaining berries on top.
Bake for 40 to 45 minutes until the top is golden. Remove from the oven and let cool for a few minutes.
Place a portion in a microwave-safe bowl with a bit of milk and microwave until warm.