Pre-heat your oven to 180°c (160°c for fan assisted oven or Gas Mark 4) and a 9"x12" inch baking tin with greaseproof paper. Set aside.
In a large bowl, cream together the margarine and sugar until light and fluffy.
Add the eggs one at time, ensuring each is mixed through before adding the next.
Sift in the flour and gently fold to create a batter.
Transfer the batter to your pre-lined tin and bake in your pre-heated oven for 50-60 minutes until risen, golden and a skewer inserted comes out clean.
Leave to cool in the tin for a few minutes before transferring to a wire rack to cool completely.
Once your cake has cooled completely; in a bowl, mix the icing sugar with a few teaspoons of boiling water, adding a little water at time until you have a thick pouring consistency. Pour the icing onto your cake, using a spoon to guide it to the edges. Finish with some rainbow sprinkles.