Preheat the oven to 200°C / Fan 180°C / Gas Mark 5. Heat the oil in a frying pan and cook the chicken for 4-5 minutes until browned. Remove with a slotted spoon and transfer into a bowl.
Add the leeks and mushrooms to the pan and cook for 2-3 minutes to soften. Remove with a slotted spoon and add to the chicken.
To make the sauce melt the butter in the same pan, then stir in the flour and gradually add the stock and cream and whisk until you have a smooth, thick sauce. Add the chicken and vegetables to the sauce, squeeze in the lemon juice and season. Mix well and spoon into 4 pie dishes (16cm x 12cm).
Roll out the ready rolled pastry and cut 4 lids to fit the pie dishes. Top each dish with a pastry lid, making sure it´s sealed around the edges using the tips of your fingers. Brush the pastry with egg and bake for 20-22 minutes until puffed up and golden. Ensure pies are piping hot and serve immediately.