Pour the stock and wine into a large frying pan. Add the garlic, thyme, tarragon and carrots, bring to the boil then reduce the heat and leave to simmer for 3 mins. Place the chicken in the stock, add some pepper, cover and simmer for 5 mins. Throw the leek slices over the chicken, cover again then gently simmer for 10 more minutes. Take off the heat and leave to cool for about 15 minutes.
Pour off the stock into a jug- you should have 500ml, add more water if needed. Add the chicken and veg into a 1.5 litre pie dish and remove the herbs. Pour the stock back into the pan, then gradually pour in the cornflour mix. Return the pan to the heat and bring to the boil, stirring continuously, until thickened. Take off the heat and stir in the crème fraîche, mustard, chopped tarragon and parsley. Season with pepper. Heat oven to 180°C fan/gas 6. Cut the chicken into chunks, pour the sauce over the chicken and combine.
Sprinkle flour on your work surface, roll the pastry to the thickness of 1/2 inch thickness. Cut a long strip wide enoughj to cover the rim of the pie dish, using your fingers fix it along the edge of the pie dish. Brush with egg, then lift the rest of the pastry on top of the pie, using the rolling pin to help. Gently press the edges with the back of a fork and trim with a sharp knife. Brush lightly with egg, then bake for 30 mins or until the pastry is risen and dark golden brown.Transfer to the oven to bake for 20-25 mins until the pastry is golden and the sauce is bubbling. Serve immediately with some lightly steamed brocolli and mashed potato if desired.