For the cake, preheat the oven to 180C/350F/Gas 4. Grease and line a deep, 20cm/8in round cake tin with baking parchment.
Break the eggs into a bowl, and lightly whisk using a fork.
Add the vegetable oil to the eggs and whisk again.
Add the grated carrots, raisins, walnut pieces and orange zest.
In a separate bowl, sift the flour, mixed spice, bicarbonate of soda and salt. Stir in the sugar.
Add the wet carrot mixture to the dry ingredients and mix well to combine, making sure there are no pockets of flour.
Spoon the cake batter into the lined tin and bake on the middle shelf for 1 - 1¼ hrs, until the cake has risen and is golden-brown all over.
Remove the cake from the oven and set aside in the tin to cool for 10-15 mins, then turn the cake out and leave to cool completely on a wire rack.
While the cake cools, make the frosting. Place the butter in a large bowl with the caster sugar, beat it for 2-3 minutes until light and creamy, then beat in the cream cheese until smooth.
Place the cake on to a serving plate or cake stand. Use a palette knife, or wide flat bladed knife, to spread the frosting over the top and sides of the cake. Scatter more walnuts on the top. It’s now ready to serve.
Before frosting, the cake will keep for a week and can be frozen for up to a month. Once the cake is frosted keep chilled.