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Heat half the oil in a large frying pan and brown the chicken for about 5 mins. Tip into a 22 x 28cm ovenproof pie dish, then evenly scatter over the ham and roast potatoes. Heat the remaining oil and soften the leeks for about 5 mins, then stir in the flour for 1 min.
Pour in the wine and stock. Bring to the boil and, stirring continuously, boil rapidly to reduce to a thick sauce. Take off the heat, then stir in the crème fraîche and seasoning. Pour over the pie filling and leave to cool.
Unroll the pastry and cut off a small strip to make decorations (if you like). Roll the remainder to fit the dish and place on top of the pie. Use a fork to crimp the edges and press the pastry to the edge of the dish. Make a small hole in the centre, then cover well with cling film. Freeze for up to 3 months. For best results, defrost at room temperature for 8 hrs.
Heat oven to 200C/180C fan/gas 6. Brush pie with egg to glaze (making sure the steam hole hasn’t sealed up). Cook for 30-40 mins, until golden and hot through.