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I keep meaning to take mine in for a regas, but haven't yet. For the Panda, I just went to Halfords as it was about the same price as buying a canister myself (about £40 IIRC). Seems like a really hard thing to fuck up. New canisrter, attach hose, connect hose to port on the car, release gas until pressure reading is about right, stop. And if they do, I'm sure it would be easy enough to get a refund or have it redone as it's a big chain. Just keep your receipt until you know it's working.
Where are you based? I've just been recommended a guy via a bmw forum for my e34, based in Woking. Let me know if you're interested and I'll send over his details