Preheat the oven to 220°C / 425°F / Gas mark 7.
Add 25g butter to a large saucepan and cook the chicken for around 5 minutes before adding the leeks.
Cook both for 4 minutes and the leeks are soft.
Transfer to a large bowl, season with salt and pepper then set aside
On a low heat, melt the remaining butter in a saucepan then add the flour.
Stir to create a smooth paste and then pour in the chicken stock and increase the heat stirring constantly.
Add the cream and reduce the heat until the sauce thickens slightly then add the thyme.
Turn off the heat and add the chicken and leeks to the sauce.
Grease a large oven-proof dish and pour in the chicken filling.
Roll out the pastry on a floured surface, to a thickness of about 1cm then layer if over the dish and trim away any excess.
Make a couple of small slits in the pastry with a knife to allow the steam to escape and then brush the pastry with the beaten egg.
Bake the pie for about 25-35 minutes until the top is golden and the filling piping hot
Remove from the oven, allow to cool before serving with some green vegetables.