In a large pot of boiling salted water, cook macaroni 1 minute less than package directions suggest. Drain well; set pasta aside.
Return ot to stove top; melt butter i pot over medium-high heat. Whisk in flour and cook 1 minute. Slowly whisk in milk; simmer until bubbly and slightly thickened. Whisk in cheeses, mustard powder, Worcestershire sauce and pepper to taste.
Add pasta to cheese sauce and cook about 1 minute until heated through. Season to taste with salt and pepper. Serve immediately.