Sautée prepared leeks in butter until just tender. Remove with slotted spoon. Add chicken to pan and seal on all sides. Add cider and half the thyme to pan, bring to boil and simmer for 5 minutes. Stir in prepared apples and cooked leeks, season with salt and pepper, then leave to cool.
Preheat oven to 220ºC (200º for fan assisted ovens)/425ºF/ Gas M7.
Divide cooled filling between dishes, then divide pastry into four equal pieces and roll to fit the specific dish size. Brush rims of dishes with beaten egg then top each dish with pastry, pressing down sides to seal. Trim off any excess.
Brush with beaten egg then sprinkle with remaining thyme and bake for 15-18 minutes or until the pastry has risen and is golden brown.