Tell them that, on the balance of probabilities, they caused the damage as everything was fine before they touched it. Ask them to put it in writing that they refuse to repair it, and then if they do inform them you'll be contacting small claims court for the costs. Go get a quote from a reputable garage, and that's your cost right there.
That's the perfect scenario. Now, whilst it's highly improbable that suddenly the pipe let go, it's also not impossible. I'd be looking at a fall back plan of them to cover the labour if you cover the parts, and obviously another regas for free.