Thank you for the help.
When it cut out from a charge by my friendly local mechanic, I thought he didn't know what he was doing so I took it to the dealer. They whacked me on the price (naturally) just to tell me it was fine and only needed to be topped off.

If I understand what you wrote, if I take it back to the dealer and complain I should expect them to try and sell me a new compressor or otherwise eat me alive?