Preheat the oven to 200C/400F/Gas 6/fan oven 180C.
Wash the leeks really well, you want a little of the water to cook them in, so they don’t need to be thoroughly drained.
Melt the butter in a sauté pan with a lid. Add the leeks and season generously with salt and freshly ground black pepper. Stir really well and then cover and cook over maximum heat for two minutes.
Stir well, and cover again, giving the pan a good shake. Cook for another two to three minutes, then remove from the heat. Place in a sieve over a bowl and allow to any excess water to drain off.
Scatter a third of the leek mixture in an even layer in the bottom of a buttered ovenproof dish that is about 25 cm/10inch in diameter. Sprinkle half the ham on top and then add another third of the leeks. Repeat until all the ingredients are used up, finishing with a layer of leeks.
Place the Cheddar in a bowl and fold in the cream and mustard, then season with the remaining pepper. Spread over the cabbage in an even layer and bake for 15-20 minutes until bubbling and golden brown. Serve straight on to the table with really crusty bread.