Finally got sick of alternately cooking myself and deafening myself by opening the car windows. Took the car to Halfords Serice Centre (yes, yes, I know) and asked them to re-gas the system - it's never worked since I bought the car 2nd hand.
Returned an hour later to be told it wouldn't take a charge. I asked if they could fix it and basically got told it's too expensive, just open your windows.
I guess it would involve a lot of pipe checking to find the leak(s)? Thus the price?