Preheat the oven to 170C, 325 F, Gas Mark 3.
Grease a loaf tin.
Peel the bananas, then mash them in a bowl then add the yoghurt and mix up.
Add the bicarbonate of soda, baking power, salt and mixed spice to the yoghurt and banana mixture and stir
In a separate bowl mix up the dark brown sugar, eggs and oil until smooth
Add the yoghurt mixture and combine.
Finally add the flour and the walnuts and mix until the flour is all mixed in.
Place in the loaf tin and cook for 45 - 50 minutes. You can check it is done by putting a skewer or knife into the banana bread and seeing
Leave to cool in the tin for around ten minutes before turning out onto a wire rack.