Make the pastry by pulsing the flour, salt and fat together then adding in the beaten egg and sufficient water to make a dough.
Divide into 2 portions of ⅓ and ⅔ then refrigerate wrapped in cling film for 30 minutes.
Poach the chicken in the stock for about 15 minutes and retain 250ml stock for the sauce and shred the chicken.
In a large pan fry the bacon in its own fat with the leeks until soft then add in the chicken.
Make the sauce by adding the 250ml of retained stock topped up with 250 ml milk to a saucepan with the flour and butter. Heat and whisk continuously until thickened and season to taste.
Pour the sauce over the chicken mixture and allow to cool.
Roll out the larger piece of dough to line a pie dish and add the meat mixture before topping with the remaining dough and sealing the edges with the beaten egg.
Glaze the pie top with egg and cook for about 30 minutes at 190 C until golden.