Grease and line a 900g loaf tin (21cm x 8cm ) with a strip of baking paper
Peel the zest of half a lemon using a potato peeler, and cut into matchstick strips. Set aside for decoration. Finely grate the rest of the zest
Make sure the margarine or butter is really soft but not melted (if not cut into cubes and leave for 5 minutes to soften or microwave for 20 seconds.
Place in a mixing bowl with just 225g of the caster sugar, the eggs, grated lemon zest and the flour. Beat well with a wooden spoon until smooth, then spread in the cake tin.
Bake for 55 minutes until golden brown and risen and the point of a knife , inserted in the centre of the cake comes out clean.
Meanwhile place the remaining 75g caster sugar with the juice squeezed from the lemons into a small pan. Place over a medium low heat and stir until the sugar has dissolved, then bring to a boil for a couple of minutes. Add the strips of lemon peel too.
Prick the warm cake all over with a skewer or fork then pour over the drizzle and leave to cool in the tin. Serve thinly sliced. The cake will keep for up to 5 days in a cake tin or can be frozen, either whole or cut into slices