Preheat oven to 220 C / Gas 7. Roll out one piece of pastry and place in a 20cm pie dish and set aside.
Place 1/2 of the butter in a large frying pan. Add the onion, celery, carrots, salt and pepper. Cook and stir until the vegetables are soft. Stir in the stock cubes and water. Bring mixture to the boil. Stir in the potatoes, and cook until tender but still firm.
In a medium saucepan, melt the remaining butter. Stir in the chicken and flour. Add the milk, and heat through. Stir the chicken mixture into the vegetable mixture, and cook until thickened. Add fresh herbs. Pour mixture into the unbaked pastry case. Roll out the top piece of pastry, and place on top of filling. Flute edges, and make 4 slits in the top to let out steam.
Bake in the preheated oven for 15 minutes. Reduce oven temperature to 180 C / Gas 4 and continue baking for 20 minutes, or until pastry is golden brown.