Hi Stuart,
Well I've just gone through an exercise myself of looking at various options for air-con. You'll obviously want something as quiet as possible so that rules out the all in one units which make a right racket.
So that probably means you'll want a reverse cycle heat-pump split system.
I've just posted some info about the system I've ordered in the thread 'Ben's new cinema room'.
As I said there, there's loads of cheap split systems on ebay, but watch out as the cheapest ones need to be professionally installed (don't know if you were planning to diy?) The next level up they come pre-charged with refrigerant and complete with pipes etc, but usually you're stuck with the exact length of pipe that they supply (about 3 or 4 metres). I also went for an inverter model as they are supposedly quieter with better temperature control.
Have a look at
http://www.global-cooling.co.uk and drop them an email and I'm sure they will be able to advise what type and size of unit would be suitable, they were very helpful when I contacted them.
Cheers,
Ben