Hi, I need to get from xingping to Hong Kong

U actually made a correct search. The way to HK is doable and convenient. And only three trains operate from Guilin to Shenzhen and they all arrive in Shenzhen North Station. However, there are direct cross-border coaches from the North Station to HK. It is realy a comfort. The direct coaches are available from 09:15 to 19:30. It has several different stops in HK, including Sheung Wan Shun Tak Centre, Mongkok Metropark Hotel and Yau Ma Tei MTR Station.

Thanks so much for that info. Do I need to book this bus in advance? If so what's the website?

Generally, it is fine to get a ticket and catch the bus on the spot. But if still wanna make it sure, search the official website of China Link Hong Kong Bus Service and try to book online.

Actually Lonely Planet says there are sleeper buses from Yangshuo to Shenzhen and Guangzhou. Does anyone know if these are running and if so what they are like? Thanks

Yes, as I know, these are sleeper buses from YS that can take you to Shenzhen or Guangzhou. The seats on the bus seem like small beds that can convenient for you to take some rest in the night. But I'm afraid they are not large enough.
To Shenzhen, it takes around 8 hours on the way.
To Guangzhou, the duration is 6 to 7 hours.

Thanks. Do you have any information on the companies who run these buses, where they go from in Yangshuo, what time etc? My hostel in xingping hasn't heard of them before.

The direct buses depart from YS Bus Station. Actually, I don't know the name of the bus company and you don't need to know it either. Just go to this bus station and take direct coach.
To Shenzhen: 12:30, 20:00 and 21:30
To Guangzhou: 10:00, 12:00, 12:30, 12:40, 19:30, 20:00, 21:00, 21:30 and 22:30