update: the dealers had the car for two days, so they could take out the battery and bench test it, etc. They immediately noticed that the cables weren't as tight as they ought to be. They could reproduce the problem without difficulty, then re-fitted everything and tightened it up, and the problem does seem to have gone. It's possible that when my car kit was fitted (not by a dealer) the fitter didn't tighten things up properly; of course, I've no evidence of that.calum said:I will see if I can get it sorted out at the dealers
Usual excellent service from Robinsons here in Cambridge: nice new 'A' class for me for two days whilst they had mine (I could have had an 'E' if I'd waited a bit longer) email updates from the service manager, no fuss, no paperwork, no cost whatsoever. Can't complain