Here in pictures is my method for putting the dish together:
1. Using butter, flour, milk and cheese, make a cheesy white sauce:
2. Cut the bacon into small pieces and lightly fry them with a little finely-sliced garlic:
3. Peel and thinly slice a couple of large potatoes. Put a layer of them into a suitable oven-proof dish, and cover them with about half of the cheese sauce. Wash and slice the Leek and add that too:
4. Add the bacon:
Here's another thought: this would work quite well with some leftover turkey or ham in it too!