Mix the dry ingredients: In a bowl mix the sugar, flour, bicarbonate of soda, mixed spice, cinnamon, nutmeg, crushed walnuts and salt until fully combined.
Mix the wet ingredients: In a separate bowl whisk together the oil and eggs, then stir into the dry ingredients with the grated carrot, courgette and orange zest until mixed.
Pour into a lined 2lb loaf tin then place onto a baking tray and bake for 50-60 minutes or until a skewer inserted comes out clean. Place onto a wire rack to cool.
For the frosting choose either the yogurt (a) or cream cheese frosting (b) to decorate the cake:
a. For the yogurt frosting: In a bowl mix the yogurt, orange zest, cinnamon and a icing sugar to taste. Spread and swirl decoratively over the top of the cake then decorate with walnuts.
b. For the cream cheese frosting: Beat the butter and cream cheese until smooth then stir in the icing sugar. Place into a piping bag then pipe onto the top of the cake, before serving decorate with walnuts.