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Heat the oven to 180C/160C fan/gas 4 and line a 900g loaf tin with parchment paper (our tin was 19 x 9 x 6cm). Put all the ingredients, except the sliced banana, 1 tbsp caster sugar and the icing sugar, into a large bowl and stir until smooth and combined.
Pour into the tin and put the two remaining banana halves cut-side up across the top of the batter, pressing down slightly. Sprinkle over the caster sugar. Bake for 1hr-1hr 15 mins until a skewer comes out clean, covering with foil towards the end of cooking if it browns too much.
Dust with icing sugar and leave to cool.