Preheat the oven to 180 C / Gas 4. Either line a large tin with grease proof paper or lightly butter the edges.
Whisk the margarine and sugar with the lemon zest until pale and thick. Beating the lemon zest will help draw out the oil making the cake lovely and lemony.
Separate the eggs, put the egg yolks in with the margarine mixture then whisk together.
Mix together the baking powder with the flour. Sieve a little of the flour and baking powder with alternate glugs of milk until all the ingredients are all well mixed. Whisk the egg whites in till they are firm and fluffy then gently mix with the other ingredients. Spoon the mixture in into the baking tin.
Bake for 1 hour 20 minutes, or until a skewer inserted into the centre of the cake comes out clean. Cool in the tin for 10 minutes before taking out of the cake tin.