My friend and I (British citizens) are currently traveling in Thailand (gap year) and looking to get a visa in Bangkok, we will be traveling overland and entering via Vietnam (11th July Approx). From there we hope to meet friends (British citizens) in Hong Kong and then travel up through China and are fly out of Beijing on 28th August (booked) back to London. We would be leaving China roughly 50 days from the day we enter.

1. Do we need a double entry visa as we are going in and out of Hong Kong? Can we get this at the embassy in Bangkok?
2. What is the duration of stay for a double entry visa? and validity? Would we be able to get it extended while in china if only 30days?
3. What documents do we require? (British citizens)

Thank you any information would be greatly appreciated)