Dip container of frozen mix into hot water just to loosen. In 2 litre saucepan, heat frozen mix, water, celery, catsup, brown sugar and Worcestershire sauce to boiling. Reduce heat; cover and simmer, stirring frequently, until mix is partially thawed, about 15 minutes. Uncover and simmer, stirring occasionally, until of desired consistency, about 10 minutes. Spoon onto bottom halves of buns; top with remaining halves.