Sorry for the late reply on this and thankyou for everyone who's replied, it's given me lots to research. I probably shouldn't have asked as vague a question as I did, but the negative feedback has been a laugh to read too.
One thing that struck me is that I don't want to be seen to be trying to take advantage of the system, or run the risk of being denied entry by leaving and re-entering too much. So it looks like I'm going to fly out to HK with my girlfriend for a week to help her settle in, come back to the UK and then fly out next year to have a six month break in HK. We'll just have to accept that that the situation will probably involve us spending some time apart. Of course marriage would solve this but I feel it would cheapen the whole thing as I wouldn't want to think I was only in it for a free holiday!
Regarding the accommodation - I guess I was using the word "Serviced" as a byword for furnished apartment - we're currently looking at options and the employer is going to help out with suggestions, and put up my GF whilst she finds a place. I realise that she might have to put up with a long (50 minute +) commute, but I think she'll only be able to know this when she's had a taste of the lifestyle. Either way we are planning something around the 16,000 for accommodation (I did see that you could get a serviced appartment for around that region here: http://www.royalview.com.hk/, although I see transport would be an issue).
I'm not planning on working whilst in HK, but will look up the various regulations so I know what's what.