Remove the sauce from the hob, add 175g/6oz of the cheddar and stir until it is well combined and melted. Add the macaroni to the sauce and mix well. Transfer to a deep suitably-sized ovenproof dish.
Remove the pan from the heat and add its contents to the pasta. For the sauce, put the butter, flour, nutmeg and mustard in a small pan set over a medium heat and cook until the butter has melted.