Preheat the oven to 180°C / 350°F / Gas mark 4.
Oil and line the base and sides of an 18cm square cake tin with greaseproof paper.
The easiest way to do this is to cut two long strips the width of the tin and put each strip crossways, covering the base and sides of the tin, with a double layer in the base.
In a large mixing bowl, combine the oil and add the eggs.
Lightly mix with a wooden spoon.
Stir in the grated carrots, raisins and orange zest.
Mix the flour, bicarbonate of soda and spices, then sift into the bowl.
Lightly mix all the ingredients until well combined.
The mixture will be almost runny.
Pour the mixture into the prepared tin and bake for 40- 45 minutes, until it feels firm and springy when you press it in the centre.
Cool in the tin for 5 minutes, then turn it out, peel off the paper and cool on a wire rack. You can freeze the cake at this point.
Beat together the frosting ingredients in a small bowl until smooth and runny.
Set the cake on a serving plate and generously drizzle the icing back and forth over the top, letting it drip down the sides.