Melt 20g of the butter in a large frying pan over a medium/high heat. Add the lardons and cook for about 5 minutes until just starting to caramelise. Add the leeks and sauté for 5 minutes or until softened. Pour in the wine and let it bubble until totally reduced. Remove from the heat and set aside to cool.
Melt the remaining 50g butter in a large saucepan over a medium-low heat, then add the flour and cook, stirring, for 1 minute to make a roux. Swap the spoon for a whisk. Slowly add the milk and chicken stock, whisking to keep the sauce smooth. Cook, stirring, for 2-3 minutes, until thickened.
Remove from the heat and stir in the cream and mustard. Add the chicken and sautéed leek mixture and season with salt and pepper to taste. Transfer to a large bowl and leave
to cool. Once cooled, stir in the frozen peas and parsley. (You can prepare the filling ahead to this point and keep it covered in the fridge, ready to assemble the pie and bake.)
Spoon the filling into a pie dish, about 28x 18cm and 5cm deep. Roll out the pastry on a lightly floured surface until 5cm larger all round than your pie dish. Brush the rim of the dish with beaten egg. Lift the pastry over the dish and let the excess overhang. Press the edges onto the rim of the dish.
Brush the pastry with beaten egg and cut a small hole in the middle, to allow the steam to escape. Place in the fridge for 30 minutes to allow the pastry to rest. Meanwhile, preheat the oven to 200°C/Fan 180°C/Gas 6.
Bake the pie in the oven for 40 minutes or until the pastry is golden and crisp. Leave to stand for about 10 minutes before slicing and serving.