There is also a chance that the HK airport runs through bus to Swatou, as far as I know there are two companies running through bus to Swatou in HK, if you make a quick search in Google, i'm sure you'll get some latest infor.
good luck!
For info.: the latest bus from SZ to ShanTou is 20:00, so you may stay in ahotel in SZ, then catch the 2nd day's bus to ST (from 8:00-20:00 daily)/
You may consider taking a bus from HK directly to ST, to save time. ($180)