Place the chicken in a large stock pot or casserole with the carrots, celery, leeks, bay, thyme, cider and just enough water to cover. Set the pot over high heat and bring it up to a simmer. Use a large spoon or ladle to skim any frothy bubbles that rise to the surface. Turn the heat down to the gentlest of simmers and pop a lid on the pan. Cook for 1 hour 30 minutes.
Meanwhile, make the pastry. Combine the butter with the flour and salt, then add just enough water to bring it together into a fairly firm dough. Form the dough into a rectangular shape with your hands and, on a well-floured surface; roll it out in one direction, away from you, so you have a rectangle about 1cm thick. Fold the two short ends into the middle so they overlap. Give the pastry a quarter turn, and repeat the rolling-and-folding process five more times. Wrap the pastry in cling film, and then rest it in the fridge for 30 minutes to an hour.
Take the chicken off the heat. Lift the chicken out of the pot carefully and let it cool. While that’s happening pass the cooking liquor through a sieve and set it aside. Be careful not to bash up the vegetables too much. Cut the carrots and celery into bite-sized chunks and set aside with the leeks.
Set a large heavy-based pan or casserole over medium-high heat. Add the butter and when it’s bubbling away, add the flour. Stir the flour into the butter to make a roux and cook it gently for a minute or so. Pour in about 500ml of the chicken poaching liquid and bring back to a simmer, stirring all the time until the sauce has thickened. Add the cream, mustard and plenty of salt and pepper. Simmer and cook gently. Once the chicken is cool enough to handle pick all the meat of the bones in shards and large chunks. Stir the vegetables and chicken into the thick bubbling sauce. Taste again and season if required. Take the pie filling off the heat and allow it to cool.
Cut the pastry into two pieces two-thirds and one-third. Roll out the larger piece and line a large lightly greased pie dish. Don’t worry if you have some overhang, this gets trimmed off. Spoon in the chicken pie filling and level it off. Roll out the smaller piece of pastry to form a lid. Brush the edges of the pastry base with some egg wash, lay on the lid and crimp together the edges. Trim away excess pastry. Brush with more egg, and cut a vent in the centre of the pie. Bake for 45-50 minutes, until golden brown. Leave to cool for 15 minutes, then serve with mash, greens and English mustard.
