You may have a leak in the system. Kwik Fit will regas and not charge if it don't cure the fault. Others have tried the aerosol DIY type refills but you can over fill and damge the system even further. Also check your pollen filter.
Just out of interest what makes you think it's not working? If your expecting the cooling fan at the front of the car to run it rarely does run. When mines on tick over you can hear the engine revs change when the AC cuts in and out.
Jim T