Without looking at the car myself and checking components/listening for abnormal noises etc... it could just need a regas, especially if it hasn't been done in a while.
There is a chance that there may be a leak though. If you have a UV torch (and UV resistant glasses for safety) shine the torch around the compressor, pipes and condenser (small radiator for the AC) If there is a leak it will glow under UV light.
Failing that, book it in to any garage that offers an AC regas service and ask them to regas the system. Part of the checks they have to do is a quick UV light check, then put the system under vacuum and check that it holds. Average cost of a regas should be about £50-75 depending on the garage and should fix the issue most of the time