Preheat the oven to 200°C / fan oven 180°C/ gas mark 6. AGA 4/3/2dr Grid shelf on the floor of the roasting oven.
Cook the macaroni according to the packet instructions and then drain it well.
Melt the butter in a medium, non-stick saucepan, remove from the heat and stir in the flour, mixing well. Now gradually add the milk, stirring all the time until it is completely incorporated. If it’s a bit lumpy, get a whisk to it!
Return the pan to the heat and bring the sauce to the boil for one minute over a medium heat, stirring continuously with a wooden spoon, until it thickens.
Remove the pan from the heat again and stir in the nutmeg, mustards, Worcestershire sauce and seasoning. Go easy on the salt though as there is plenty in the cheese. Stir the cheeses into the sauce, reserving one tablespoon of the Cheddar.
Stir the cooked macaroni into the sauce to coat it thoroughly. Spoon the macaroni cheese into a 1.2 litre/2 pint ovenproof dish. Place the sliced tomatoes on top and sprinkle with the reserved Cheddar cheese. Bake in the oven for about 20 minutes until golden and bubbling.
Serve with crusty bread and a green salad.