Put the bacon into a large, cold frying pan, and cook over a low heat until golden and starting to crisp up. Scoop out of the pan with a slotted spoon, set aside, then add the leeks and butter. Cook until softened, about 10 minutes, then stir in the flour until the leeks have completely absorbed it.
Gradually stir in the stock, then simmer until it thickens to a sauce consistency. Stir in the crème fraîche, grainy mustard and English mustard. Take off the heat and season.
Tip the leek sauce, chicken and crispy bacon into a deep baking or pie dish with a lip.
Dampen the lip of the dish with some water. Unroll the pastry sheet and roll out a little thinner if you need it bigger, then lift carefully onto the pie. Trim excess by running a sharp knife along the edge of the baking dish. Brush the top with beaten egg, then scatter with some salt and freshly ground black pepper. Chill until needed or bake immediately.
Preheat the oven to 200°C (180°C fan oven) Gas Mark 6. Poke a steam hole in the centre of the pastry and then bake for 30 minutes, or until the pastry is golden.