Chicken and Leek Potato Topped Pie

I always make this on a Monday or Tuesday with leftovers from a Sunday roast, it's great as it uses not only the meat, but any vegetables and gravy too.
This can be topped with creamy mashed potato or the slices which I've done here, either works well.  There is also no reason why you can't top it with pastry, particularly easy with bought puff pastry!

Ingredients

Approx. 1 kg Potatoes, peeled and thinly sliced
1 pint of Chicken Stock

3 Large Leeks, sliced and washed thoroughly
2 cloves of Garlic, chopped or minced
 Knob of Butter + Drizzle of Olive Oil
Splash of White Wine
1 tspn Dried Sage (optional - fresh thyme is also good)
2 ladles of chicken stock (out of the pint above)
Any left over gravy (if none put in 3 ladles of stock)
Any left over veg (I always have lots of carrots which go well)
1 heaped tspn Dijon Mustard
A tablespoon or two of Cream (optional)
Plain flour mixed to a paste with butter to thicken (if needed)
Salt and Pepper
Cooked chicken from about half a roast chicken, chopped or torn into chunky pieces
Oil to brush on the top

• Bring the stock to the boil, add the potato slices and simmer for 4 minutes.  Carefully take the potato slices out of the stock and set aside.  This stock can be then used again in the filling.
• In a large pan melt the knob of butter with the oil, add the leeks, garlic and splash of white wine then sprinkle in the sage (or other herb you like).  Give it a good stir then turn the heat down to minimum, cover and cook slowly for 20 minutes.  After this time the leeks should be soft, silky and sweet.  Pre-heat the oven to 175C.
• Add to the leeks the gravy (if you have it), 2 ladles of stock, the leftover veg (if you don't have any just boil or steam some chunky cut carrots until tender and add them) and the mustard.  Turn the heat up, add the cream (if using) and give it all a stir to combine, check the seasoning at this point and add salt and pepper to your taste.
• If it needs thickening, add the flour/butter mixture a teaspoon at a time stirring well until the sauce is as thick as you want it, give it a few minutes bubbling to cook the flour.
• When you are happy with the seasoning and the thickness of the sauce, turn off the heat and gently stir in the cooked chicken.
• Transfer the filling to an oven proof dish and layer the top with the half-cooked potato slices, brushing them with oil and seasoning with salt and pepper.
• Put uncovered into the oven for about an hour or until the filling is bubbling around the edges and the potatoes are browned and tender when poked with a sharp knife.