If your aircon's not working, the most likely cause is loss of gas. This could be due to natural cause, such as leaking out through seals, but nowadays is more often because of condenser (radiator) stone damage.

A respectable place would place a dye in the new gas that can be detected by (UV?) light in the case of a leak.

Do not rule out a simple electrical fault, preventing the aircon working, or the compressor clutch, or...................................................................

My current 4-year-old car has aircon that's below comparison with its 11-year-old replacement. I had the old one re-gassed at nine years and the report showed it had lost around 10% of its gas. I've had the current car for four years and I've never been impressed by the aircon efficiency. I won't put it in for an aircon service until it fails to meet my requirements. As it still cools, i doubt the problem's a gas leak, but more likely its effectiveness, or a lack of gas at new.

I'd also rule out either of the options the OP's suggested, but go to a an aircon specialist.

Edited by bathtub tom on 21/04/2018 at 23:53