If you're lucky you'll get away with a simple regas. Depending where you go this should cost £50 - £90 this time of year. If you regas it, it's cold again for a few hours then goes hot again you've got a leak!
I had this on my GT, the high pressure pipe coming off the compressor had a crack and needed to be replaced. The part was about £70 odd. It's possible to fit yourself however the front bumper has to come off. It would probably take a garage 2hrs and £350 - £400 to do the job. If you're a well meaning amateur give yourself a good 4 hrs. It's not that hard, it just takes a bit of time if you haven't done it before. Best of all you'll be saving yourself a few hundred notes in the process.