Cook the macaroni to packet directions. Drain well, shaking the macaroni to remove as much water as possible. While the macaroni is cooking, melt the butter in a saucepan. Remove from the heat and mix in the flour until smooth. Return to the heat and cook stirring until the mixture is frothy. Mix in the mustard. Remove from the heat and mix in a little milk at a time, stirring after each addition until smooth. Return to the heat and cook until the sauce boils and thickens. Mix in the salt, cheese then the drained macaroni. Spoon into a serving dish and grill until lightly golden.