I was there for 12 hours a couple years ago. I am fairly certain you do not need a visa if you stay inside the airport. I would suggest you download movies/take dvds with you and kill your time. Also, if you don't have one already, buy a UK/Hongkong prong adapter, since the American ones don't work there.  I forgot to take mine on my way home, and my laptop died within the first couple hours. They are fairly cheap.