Put the chicken into a deep pan that is just large enough to hold it quite snugly. Pour in enough water to half cover the bird, then tip in the peppercorns, bay leaves, thyme, onion and celery. Add the salt and bring to the boil on the hob, then reduce the heat, cover tightly and simmer for 1-11⁄4 hours until tender.
Transfer the chicken to a plate. Strain 425ml/3⁄4 pint of the stock into a measuring jug (top up with water if you need to). Strip the meat off the chicken, cut into bite-sized pieces and put in a 1.7 litre/3 pint pie dish.
Put the eggs in boiling water and boil for 6 minutes. Drain, cool them under cold water and remove the shells. Quarter the eggs and nestle them in the pie dish among the chicken pieces.
Heat the butter and oil in a large frying pan and fry the bacon until crisp. Add the leeks and cook for 2 minutes until softened. Stir in the flour and cook for 1 minute. Pour in the chicken stock a third at a time, stirring continuously and making sure the sauce is thick and smooth before adding the next batch. Stir in the mustard, crème fraîche and parsley, then add a squeeze of lemon juice and black pepper to taste. Add salt if needed. Spoon over the chicken and leave to cool.
Meanwhile, preheat the oven to 220C/Gas 7/fan 200°C Reserve a quarter of the pastry for decoration and roll out the remainder to a shape about 5cm/2in larger than the top of the pie dish, then cut a 2.5cm/1in strip from all round the edge. Brush the edge of the dish with water and stick the pastry strip to it. Brush the strip with water. Flip the pastry lid over the rolling pin, lift it up and unroll over the pie. Press the edge on to the pastry strip to seal, and trim any excess with a sharp knife.
Tap the blade of a small sharp knife all along the outside edge of the pastry (this helps the edge to rise during cooking), then flute it using the back of the knife. Make a hole in the lid centre with the tip of the knife, to allow steam to escape as the pie bakes.
Roll out the pastry you put aside earlier. Cut 8 long strips each about 2.5cm/1⁄2in wide, and two more strips each about 4cm/11⁄2in wide. Cut the wider strips diagonally to make eight diamond leaf shapes. Mark the veins of the leaves with the tip of a sharp knife. Brush the pastry lid with beaten egg, then lay the 8 thin strips on top, as in the picture. Brush again with egg and arrange the leaves in the centre, between the strips. Brush with egg.
Chill the pie for 15 mins, if you have time, to set the pastry. (Can be made a day ahead up to this point. Keep chilled and add 4-5 extra minutes at baking time.) Bake for 30 - 35 miutes until the pastry is crisp, puffed up and deep golden brown.).