Preheat the oven to 180°C, 350°F, Gas 5.
Cook the potatoes in boiling salted water until tender. Drain thoroughly, then mash with butter until completely smooth. Season and set aside.
Melt the butter in a pan and add the onion, garlic, leeks and celery and cook for about five minutes until softened. Stir in the flour and cook out for two minutes, then pour in the stock, a little at a time, stirring constantly until all the stock is incorporated. Add the bay leaf and leave to simmer gently for 10 minutes, until the sauce is smooth and thick.
Stir in the chicken and chopped parsley and season. Spoon the chicken mixture into the base of a baking dish, then top with a layer of mashed potatoes. Sprinkle the grated cheese over the potatoes and bake in the oven for 25-30 minutes until the cheese is golden and the sauce is bubbling. Ensure meat is cooked through with no pink showing before serving.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.