Preheat the oven to gas 6, 200ºC, fan 180ºC. Heat the oil in a large nonstick frying pan and cook the chicken and leeks for 4-5 minutes, or until the chicken is browned.
Sprinkle over the flour and mix well. Cook for 1 minute before stirring in the stock. Bring to the boil, stirring continuously, for about 3 minutes or until thickened. Turn down and simmer for another couple of minutes before stirring in the cream and mustard. Divide between 4 small pie dishes.
Roll the pastry out and cut into 4 pieces big enough to cover the top of each pie. Brush the edges of the pie dishes with the beaten egg and top each with the pastry, pressing to stick down. Glaze the pies with the rest of the egg and make a hole in each for steam to escape. Bake in the oven for 20 – 25 minutes until golden.