Preheat the oven to 180c/350f/gas 4. Grease the tin/tins.
Finely grate the lemon zest and squeeze the lemon juice into a small bowl. Cream the margarine and sugar together in a bowl until smooth. Beat the eggs and beat them into the creamed mixture.
Sift the flour and baking powder and add gradually to the creamed mixture, along with the finely grated lemon zest and the lemon juice. Add sufficient milk to give a soft dropping consistency. Spoon into the prepared tin.
Bake in a preheated oven for 30 minutes. Remove from the oven and leave to cool in the tin.
For the icing, squeeze the juice from the lemons and blend this with the icing sugar and enough water to form a good spreading consistency. Spread over the sponge when it has cooled.