While macaroni is cooking, melt butte in a 3qt. sauce pan over low heat. Stir in flour, salt, pepper, mustard and worcestershire sauce. Cook over low heat, stirring til mixture is smooth and bubbly, remove from heat. Stir in milk. Heat to boiling, stirring constantly.Boil and stir 1 minute.
Stir in cheese cubes. Cook stirring until cheese is melted.