Cook the macaroni in a large saucepan of boiling salted water for 8-10 minutes; drain well and set aside.
Melt the butter over a medium heat in a saucepan slightly larger than that used for the macaroni. Add the flour and stir to form a roux, cooking for a few minutes.
Gradually whisk in the milk, a little at a time. Cook for 10-15 minutes to a thickened and smooth sauce.
Meanwhile, preheat the grill to hot.
Remove the sauce from the stove, add 175g of the cheese and stir until the cheese is well combined and melted.
Add the macaroni to the sauce and mix well. Transfer to a deep suitably-sized ovenproof dish.
Sprinkle over the remaining cheddar and the parmesan and place the dish under the hot grill. Cook until the cheese is browned and bubbling. Serve hot with a side salad or fresh vegetables.
