Place the macaroni in a large, microwaveable bowl and add boiling water until it's just covered. Place in the microwave and cook at the highest setting for 3 minutes, then stir and microwave for another 2 minutes.
Carefully drain the pasta, then return to the bowl. Stir through the butter and, once it's melted, add the milk, cheese (reserving a small handful of red Leicester), cornflour, frankfurters, tomatoes and mustard powder. Season to taste, stir well and divide between 4 large mugs, then sprinkle over the remaining red Leicester.
Place all 4 mugs in the microwave and cook for 10 minutes on the highest heat.