Preheat the oven to 180 C / Gas 4. Grease a 22cm x 33cm cake tin and line with baking parchment.
In a bowl, combine the flour, sugar, baking powder and bicarb and mix well. Add the grated carrot, oil, eggs, cinnamon, raisins and walnuts and beat until well incorporated. Spoon the mixture into the prepared cake tin.
Bake in the preheated oven for 40 to 45 minutes, or until a skewer inserted into the centre comes out clean. Remove from the oven and transfer to a cooling rack to cool completely.
For the cream cheese icing:
Cream the butter (if you use margarine the icing
will be softer) and cream cheese together. Add sugar and milk and using a handheld mixer, beat on high speed until smooth and creamy.
Transfer the cake to a serving plate. Thin a little with a splash or two of milk to base ice the cake, and use full strength to do decorative borders. Slice and serve.