Remove skin from chicken. Cut flesh into 2.5 cm cubes.
Heat oil in a large saucepan. Add bacon, mushrooms, onion and garlic. Cook until onion is clear. Stir in flour and cook until frothy.
Gradually add stock and bring to the boil. Add chicken, herbs, milk, salt and pepper to taste. Reduce heat and cook gently for 20 minutes or until juices run clear when tested, stirring occasionally. Stir in corn. Remove from heat and allow to cool.
Preheat the oven to 220˚C. Pour chicken mixture into a 20cm pie dish. Brush edge of dish with water.
On a lightly floured board cut pastry to a circle large enough to fit top of pie dish. Carefully place pastry over filling. Press edges firmly to seal, then trim. Decorate pie with pastry trimmings if desired. Cut steam holes in centre of pastry top. Brush pastry with egg yolk.
Bake for 20 minutes or until pastry is golden and well risen.