Sauté 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.
Divide cooled filling between 12 individual pie dishes. Brush rims of dishes with beaten egg then top each dish with a pastry round, pressing down sides to seal. Brush with beaten egg then sprinkle with remaining thyme.
Bake for 15 - 18 minutes or until the pastry has risen and is golden brown.