Sift the flour and baking powder into a roomy mixing bowl, lifting the sieve quite high to give the flour a good airing as it goes down, then add the butter, lard, egg and caster sugar.
Now, using an electric hand whisk, mix to combine all the ingredients for about one minute until you have a sandy texture. Then in a separate bowl mash the bananas to a pulp with a large fork and briefly whisk them into the cake mixture. Now fold in the chopped walnuts and orange and lemon zests.
Spoon the cake mixture into the prepared tin, level it off on top with the back of the spoon and sprinkle with the demerara sugar. Bake on a lower shelf so the top of the tin is aligned with the centre of the oven for approximately 1 hour 10 minutes.
Leave the cake in the tin for 10 minutes, then turn it out onto a wire cooling rack.
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