Put a saucepan with the water onto boil, and then add all the macaroni.
Leave the macaroni to simmer for as long as the pasta bag says to (Usually around 12 minutes) but do not cook thoroughly as this will happen when baked.
Get a smaller saucepan and mix the butter and flour until combined and dough like (do this on a low heat) and then slowly add the milk, a little at a time.
When all the milk is mixed in, add the mustard powder and slowly add the cheese, a handful at a time. You will know when the sauce is done because it will have thickened. Add as much seasoning as you want.
Drain the pasta and put the sauce on top. Add a little grated cheese and bake for around 7 minutes on high heat (around 180 degrees or gas mark 6/7) until the cheese on top has melted and gone golden.