1. Poach the frankfurters for 5 - 6 minutes in slightly salted boiling water.
2. Remove the skins from the frankfurters and, when cold, slice the meat diagonally
3. Cook the macaroni in plenty of boiling salted water for about 20 minutes, or until tender.
4. Rinse in cold water and drain well.
5. Melt the butter in a saucepan. Stir in the flour and cook for 1 minute.
6. Remove the pan from the heat and add the milk gradually, beating thoroughly and returning the pan to the heat to cook between additions. When all the milk has been added, simmer for 2 minutes, stirring occasionally
7. Stir the frankfurters, grated cheese and mustard into the sauce mixture. Season to taste.
8. Add the drained macaroni to the sauce and frankfurter mixture, and stir well until heated through.
9. Pour the mixture into an ovenproof dish and sprinkle the top with a little extra grated cheese, if desired.
10. Cook the macaroni under a preheated moderate grill, until the top is golden brown.