will we be allowed another 72 hour visa for our time in Shanghai?

In February my wife and I will be flying from London Heathrow non stop to Shanghai and will land approx at 0925 local time at 1530 the same day we will fly out to Saigon. So can we leave the airport rather than sit around for that amount of time? On Tuesday the 17th Feb we will fly from Siem Reap into Hong Kong then catch a connecting flight to Shanghai we will then stay in a Shanghai Hotel Tuesday night and Wednesday night then catch our flight on Thursday 19th back to the U.K. So We are British passport holders. Any help would be brilliant thanks