For the pastry, put the flour and salt in a large mixing bowl. Add the margarine, mix until it resembles breadcrumbs. Add the cold water and stir until the pastry comes together to form a dough. Put it in the fridge to rest until ready to use
Season and add a little oil to the chicken and cook in the oven
Chop the leeks and wash really well, sweat off in a little oil until soft but not coloured
Melt the margarine in a saucepan then add the flour and cook, stirring until sandy in colour and texture. Add the milk, whisking all the time. When the sauce has thickened, add the mustard and chopped parsley, put to one side
Stir the cooked chicken and leeks into sauce, check seasoning
Divide the pastry into eight pieces. On the floured side roll on e piece out to line the base of your pie tin
Put the chicken and leeks into the base, roll out the remaining pastry to form a lid for the pie. Dampen the edges of this and the pastry in the base, and place on top. Press the edges together. Brush the top with milk and make a slit in the middle of the pastry to let steam out.
Bake in the pre-heated oven until golden brown for 30-35 minutes