In a large saucepan of boiling water, cook the macaroni for 8-10 minutes.
In another saucepan, melt the butter over a medium heat and slowly add the flour. Stir to form a roux and keep cooking for a few minutes.
Gradually whisk in the milk a little section at a time and cook for a further 10-15 minutes until thickened and smooth.
Meanwhile, preheat the grill to hot.
Remove the sauce from the hob, add 175g of the cheese and stir until it is well combined and melted.
Add the macaroni to the sauce and mix well. Transfer to a deep 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 straightaway.