Method:
Rub the margarine and flour together in a bowl until you have a mix which is similar to breadcrumbs. Stir in the sugar. Spread the mixture evenly into a 9" (23cm) square tin which has been lined with baking parchment. Bake in a pre-heated oven at 170C/340F (160C/320F if fan assisted) for approximately 35 minutes until it is golden brown. Allow the base to cool.
Heat the filling ingredients together in a pot, making sure that you stir it constantly (otherwise it will stick!) until it begins to simmer. Continue stirring until it thickens (which it should do in a few minutes). Spread the filling evenly over the base and again allow to cool.
Melt the chocolate so that you can spread it over the filling.
When it has cooled and you are ready to eat it, cut up into squares or rectangles with a sharp knife.