Preheat the oven to 180°C (350°F).
Use one sheet of pastry to line the bottom of an 8 inch (20 cm) pie dish.
Meanwhile, melt the butter in a frying pan over medium heat.
Stir in the leeks, and cook until they begin to soften.
Add the chicken and cooked vegetables.
Sprinkle with the flour and stir through.
Add the milk and stir until a smooth sauce is formed.
Cook for a minute or two.
Remove from the heat and allow to cool for ten minutes.
Pour the filling into the pastry dish.
Brush the edge with beaten egg and top with the remaining sheet of pastry.
Cut two small slits in the top of the pastry to allow the steam to escape.
Brush the pastry with beaten egg.
Bake the pie for 20 to 25 minutes until the pastry is golden brown.