Preheat the oven to gas mark 4/350oF/180oC/160oC fan and line a rectangle cake tin that measures approximately 13 x 9 inch (33 x 22cm). To a large bowl, add and combine the self-raising flour, salt, nutmeg, cinnamon, ginger and bicarbonate soda then set aside.
To a large mixing bowl, add the vegetable oil, vegetable spread, brown sugar and caster sugar. Cream them together until well combined then mix in the eggs, vanilla extract and yoghurt.
Pour in the dry ingredients set aside earlier then add in the grated carrots. Once all the ingredients are well combined, pour the batter into the prepared baking tin and bake for 40 minutes or until cooked throughout. Once baked, place the baking tin on a wire rack to completely cool.
Make the frosting by adding the butter to a mixing bowl and beat for 40 seconds using a paddle attachment on a medium setting. Add in the cream cheese and continue to mix for around 1 minute until creamy and fluffy. Gradually add in the icing sugar and mix until you can no longer see any icing sugar. Spread the cream cheese icing onto the cooled cake then sprinkle on the chopped walnuts. Place in the fridge for around 30 minutes to help the frosting set then cut and serve.