Fry the bacon in the butter; drain put to one side.
In the bacon juices add the leeks and a bit more butter if needed. Cook until soft. Put with bacon when done; tip the juices over.
Make the white sauce: Melt butter then stir in flour. With a whisk add the milk; keep on the heat and whisking until all the lumps have gone and thickened up. Season with salt and pepper.
Add the cream and more milk if too thick. If too runny in a cup mix some more flour and milk making sure there are no lumps and add bit by bit until thicker.
Add the cooked leeks and bacon; stir in. Add the cheese and stir until melted.
Tip half the sauce into an ovenproof dish. Place the chicken breasts on top of the sauce. Tip the rest of the sauce over the chicken. Cover with foil.
Cook in a preheated oven at 190 degrees C for 30 minutes.