Place chicken breasts in a pot of cold water and bring to a boil. Turn down the heat and simmer for 15 minutes, until chicken is cooked. Once cooked remove from water and allow to cool, before shredding. Keep the poaching liquid.
In a very hot pan flash fry the mushrooms in a little olive oil until golden brown. In the same pan, sauté the leeks until they are soft, but still retain their colour. Again in the same pan melt your butter and add your flour to make a roux. Remove pan from the heat, and slowly start adding 500ml of the poaching liquid. Return to the heat and cook, stirring continuously until the sauce starts to thicken. Add the chicken, mushrooms, and leeks to the sauce, as well as the cream. Season well with salt and pepper and allow to simmer for a further 10 minutes
Allow to cool completely. Preheat oven to 200°C. Place the cold filling into a pie dish with the pie bird in the middle. Brush the edges of the pie dish with egg so that the pastry will not stick. On a floured surface, roll out the puff pastry, until it is big enough to cover the pie dish. Place the pastry over the pie, and cut an opening for the bird. Using a fork press down the edges. Brush the pastry with egg and place in the oven. Bake for 45 minutes or until dark golden brown.