1. Drain the anchovies, and set enough aside to slice to make a thin lattice over the dish. Chop the rest finely.
2. Cook the macaroni in plenty of boiling, salted water for 10 minutes, or until tender but still firm. Rinse in hot water and drain well.
3. Meanwhile, melt the butter in a pan. Stir in the flour and cook for 1 minute.
4. Remove from the heat, and gradually stir in the milk. Return to the heat and bring to the boil. Simmer for 3 minutes, stirring occasionally.
5. Stir in the mustard, anchovies, and half the cheese. Season with salt and pepper to taste. Stir in the macaroni, and pour into an ovenproof dish.
6. Sprinkle the remaining cheese over the top, and make a latticework with the remaining anchovies. Brown under a hot grill. Serve immediately.