For the icing
If you have time to do it the night before, put the orange zest and juice in a bowl with the sultanas. If you don’t have time, on the day simply stir the zest, juice and sultanas together and microwave on Medium for 1-2 mins.
Heat oven to 180C/160C fan/gas 4 and grease and line the base and sides of a 2lb loaf tin. Whisk together the oil and eggs. Mix together the sugar, flours, mixed spice, cinnamon and bicarb in your largest mixing bowl. Add the sultanas with any juice and zest left in the bowl, grated carrot, walnuts and whisked egg mixture into the dry ingredients, then thoroughly mix with a wooden spoon. Tip into the loaf tin and bake on the middle shelf for 1 hr, or until a skewer comes out clean. Cool the cake in the tin.
Once cool, remove from the tin and make the icing. With an electric whisk, beat together the soft cheese, butter, icing sugar and cinnamon until smooth. Spread over the top of the cake and decorate with walnut halves.