Peel then mash the bananas with the lemon juice.
Put the flour, baking powder and sugar in a mixing bowl. Add the mashed bananas, melted butter and eggs and mix together. Stir in the dates and walnut pieces then spoon into a greased 1 kg (2 lb) loaf tin, its base and 2 long sides also lined with oiled greaseproof paper. Spread the surface level and decorate the top with walnut halves and banana chips, if using.
Bake in the centre of a preheated oven, 160°C (325°F), Gas Mark 3, for 1 hour 10 minutes–1¼ hours until well risen, the top has cracked and a skewer inserted into the centre comes out clean. Leave to cool for 10 minutes then loosen the edges, turn out on to a wire rack and peel off the lining paper. Leave to cool completely. Store in an airtight tin for up to 5 days.