Melt butter or margarine over low heat; stir in flour and remove from heat. Add milk, stirring constantly. Return to stovetop; over low heat cook, stirring constantly, until mixture is thickened. Stir in cheese and continue coooking until cheese is melted. Pour milk and cheese mixture over macaroni and mix well. Turn into a lightly greased baking dish. Mix bread crumbs with melted tablespoon of margarine. Sprinkle with buttered crumbs. Bake for 30 minutes at 180ºC until top is golden brown.