Sift the flour and salt into a large mixing bowl. Add the margarine and rub into the flour until the mixture resembles breadcrumbs.
Sprinkle with cold water a tablespoon at a time, and mix in using a knife. Add a drop more water only if the dough will not clump together. With your hands, gather together into a firm but pliable dough, handling as little as possible.
Wrap the ball of dough in cling film and chill for at least 30 minutes before rolling out.
Line a flan or tart tin with the rolled out pastry.
Mix all filling ingredients together and pour into the pastry case.
Bake at about 180 C / Gas 4 for about 25 minutes or until the mixture is set.