Preheat the oven to 180 C / Gas 4. Lightly grease with some butter and cover with baking parchment 2 sandwich tins.
Cream the margarine in a large bowl until light and fluffy. Add the caster sugar to the margarine and cream together until all mixed together.
Crack all 3 eggs into another bowl and lightly whisk together. Add the beaten egg a little at a time untill all the egg has been added to the mixture.
At this stage add the grated lemon zest to the mixture, and half the juice of the lemon.
Now sift the flour into the softened mixture until all mixed together. Evenly spread the mixture between the two prepared tins.
Bake in the preheated oven on middle shelf until lightly golden brown and springs back when pressed, about 15-20 minutes. Leave to cool on a cake rack.
When both cakes are cooled, spread the lemon curd over one half of the cake, and place the other half of the cake on top.
Now sprinkle caster sugar on top of the cake lightly, and squeeze the juice of the other half of the lemon, all over the top of the cake, until you cant see the white of the sugar. Leave for an hour before eating.
Can be wrapped in brown paper and frozen on day. It will keep in the freezer for 3 months. Must defrost before eating.