Thanks David for the info.
I'm going to Guangzhou via Shanghai.
The train from Guangzhou to Changsha is only 2 hours and 20 minutes, and is 327 yuan.
I check cheapflights all the time, but Cathay always seems to be expensive. I wish I could find a flight to Changsha for £499. Anyway, I've already booked it. I know what you say about waiting for cheaper tickets, but I think I got a pretty good deal.