Chicken leek and mushroom pie

Name: Chicken leek and mushroom pie
Prep time: 20 min
Cook time: 40 min
Total time: 1 h
Cuisine: European, British
Ingredients: 2 baking potatoes (about 450g/1lb total weight) washed well (peel left on) and cut into chunks; 350ml (12fl oz) skimmed or semi-skimmed milk plus 1-2 tbsp for the potatoes; 1-2 tbsp snipped fresh chives; Freshly ground black pepper to taste; 2 tsp rapeseed oil; 2 small leeks washed and sliced; 115g (4oz) chestnut mushrooms sliced; 2 tbsp cornflour; 2 tsp wholegrain mustard or to taste; 1 tbsp chopped fresh parsley; 115g (4oz) cooked skinless boneless chicken breast chopped