Pack the chicken thighs into a saucepan, pour over the cider and stock, then season. Cover and simmer for 45 minutes.
Lift the chicken on to a plate, and simmer the leeks in the stock for 4–5 minutes. Strain the leeks, reserving the stock in a measuring jug. Make up the stock to 600 ml (1 pint) with water, if needed.
Wash and dry the pan, then melt the butter in it. Stir in the flour, then gradually whisk in the stock and bring to the boil, stirring until thickened. Mix in the herbs and season.
Dice the chicken, discarding the skin and bones. Put into a 1.2 litre (2 pint) pie dish with the leeks. Pour over the sauce.
Roll out the pastry on a floured surface until a little larger than the top of the pie dish. Cut 4 strips about 1 cm (½ inch) wide and stick along the rim with a little egg. Brush the top of the strip with egg, then press the pastry lid in place. Trim off the excess and crimp the edge. Cut leaves from the excess and add.
Glaze the pastry lid and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 30 minutes until golden.