Preheat the oven to 200°C / 180°C fan / Gas 6. Line a 2lb loaf tin with baking parchment.
In a large bowl, combine the self-raising flour, baking soda, wholemeal flour, brown sugar, oats, ground almonds, walnuts, mixed spice, and salt.
In a small saucepan, gently melt the butter and treacle together.
Mash the bananas in a jug, then whisk in the egg, yogurt, vanilla essence, and buttermilk.
Add the wet mixture and the melted butter–treacle mixture to the dry ingredients. Stir gently with a wooden spoon until well combined.
Combine the crumble mixture and sprinkle it on top.
Pour the batter into the prepared tin. Bake for 1 hour 40 minutes, covering the top with foil after about 45 minutes once golden.
For a crisp crust, remove the loaf from the tin and foil in the last 10 minutes and return to the oven.
Allow to cool before slicing.
