Preheat the oven to 180C (160C fan, 350F). Grease a 1 lb loaf tin and line the base with non-stick baking paper. Place the dates, bicarbonate of soda and lemon rind in a bowl and add the hot tea. Leave to soak for 10 minutes, until soft.
Cream together the butter and sugar until light and fluffy, then beat in the egg. Stir in the date mixture.
Fold in the flour using a large metal spoon, then fold in the walnuts. Spoon the mixture into the prepared cake tin and spread evenly. Top with the walnut halves.
Bake in the preheated oven for 35-40 minutes or until risen, firm and golden brown. Leave to cool for 10 minutes in the tin, then turn out and finish cooling on a wire rack.