Mark made this really well in Norfolk (2012) and at the Cottage (2013).
I don't use a recipe to make this any more, but it's vaguely based on a delia original.
Make a roux cheese sauce, about 50g of butter for 4 people I think, and then however much flour and milk you need........excess being the order of the day cheese wise, I like a decent mature cheddar.
Fry lardons of smoked streaky bacon, then add leeks and mushrooms (and/or). Fresh thyme makes a lovely addition - absolutely best buddies with mushrooms. When they are all sauteed throw it together with pasta, cover with cheese and shove in the oven. A recent change I have made is just to really stick it under the grill to gratinate, rather than actually bake in the oven.....this avoids the sauce drying out and overcooking the pasta.