Preheat the oven to 200C/400F/Gas 6.
Sieve the flour and salt into a bowl.
Rub in the margarine or butter until the mixture resembles fine breadcrumbs.
Add the cold water to the flour mixture. Using a knife, mix the water into the flour, using your hand to firm up the mixture. The pastry should be of an even colour and suitable consistency for rolling.
Divide the pastry into two halves. Take one half and roll it out so that it is big enough to cover an 20cm/8in enamel or aluminium plate. Trim the edges with a knife using the edge of the plate as your guide.
Cover the pastry with the stewed apples and sprinkle with sugar to taste.
Roll out the other half of the pastry. Moisten the edge of the bottom layer of pastry and place the second piece on top.
Press down on the pastry edges, making sure that they are properly sealed. Trim off any excess pastry with a knife in a downward motion, again using the plate as your guide.
Flute the edges with a pinching action using your fingers and thumb.
Prick the surface of the pastry lightly before placing the pie in the oven. Cook for 20-30 minutes.
When the pie is cooked it should move slightly on the plate when gently shaken.
Slide on to a serving plate, dust with caster sugar and serve.