Bring a large pan of water to the boil, cook the leeks for 8-10 minutes or until tender.
Melt the butter in a small pan, add the flour. Gradually stir in the milk, mixing to prevent lumps.
Once it has all been added simmer for 3-4 minutes.
Add three quarters of the cheese and Tracklements Strong English Mustard and simmer for 3-4 minutes, season to taste.
Preheat the grill to high. Drain the leeks thoroughly, wrap each one in 2 slices ham and sit side by side in a greased ovenproof dish. Pour over the sauce.
Mix the remaining cheese and breadcrumbs together and sprinkle over the top. Grill for 5 minutes or until brown. Serve with crusty granary bread or jacket potatoes.