Preheat the oven to 200°C / 400°F / Gas mark 6.
To make a bouquet garni, use a piece of string to tie together a couple of rosemary, thyme and parsley sprigs and a handful of bay leaves.
Put the chicken in a large pan, to hold snugly.
Add the carrots, onion and celery tucking them in around the chicken.
Add the bouquet garni and pour in enough water to come just over halfway up the chicken, season with salt and pepper.
Bring to the boil, reduce the heat and cover tightly.
Simmer gently for 1-1¼ hours, until the chicken is cooked through.
Carefully lift the chicken out onto a baking tray or deep dish to catch any stock that leaks out.
Strain the stock, discarding the veggies and bouquet garni, and measure 500ml into a jug. Any extra can be cooled and frozen for another time.
Strip the meat from the chicken, discarding the skin and bones.
Cut the meat into large chunks and put into a large pie dish.
Melt the butter in a pan, add the leeks and fry until it starts to soften.
Add in the flour and cook for 1 minute stirring.
Gradually stir in the stock, cooking until the sauce is smooth and no floury lumps are present.
Remove from the heat and add lemon, parsley and crème fraiche. Season with salt and pepper.
Pour the sauce over the chicken and leave to cool.
Roll out the pastry and trim to 5cm larger than the pie dish.
Brush the edge of the dish with water.
Lay the pastry on the pie, tucking the edges under to make a double layer around the rim.
Press the pastry edge to seal it.
Brush the pastry with milk or egg and bake for 30-35 minutes until the pastry is golden.