Heat oven. Heat the oven to 180°C (160°C fan, gas mark 4). Put the shelf low, so the top of the tin sits level with the middle of the oven. Line a 2lb loaf tin.
Prep flavourings. Finely grate the orange and lemon zest. Roughly chop the walnuts.
Sift dry ingredients. Sift the flour and baking powder into a large bowl, holding the sieve high.
Add fat, egg, sugar. Add the butter, lard, beaten egg and caster sugar to the bowl.
Whisk until sandy. Whisk with an electric hand whisk for about 1 minute until the mix looks like damp sand.
Mash bananas. Mash the bananas to a pulp in a separate bowl. Whisk them briefly into the mix.
Fold in. Fold in the walnuts and zest until just mixed.
Fill tin. Spoon the batter into the tin, level the top and sprinkle with the demerara sugar.
Bake loaf. Bake for about 1 hour 10 minutes, until a skewer in the centre comes out with no wet batter. Start checking at 1 hour in a fan oven.
Cool down. Leave in the tin for 10 minutes, then turn out onto a wire rack. Cool for at least 30 minutes before slicing.