Just got back from Tesla. They gave good service.
They said they checked the 12V battery and the logs say that it never went below 12.4V so they just disconnected it and then did a "Gateway reboot" and the error message went. So it was put down to a "software glitch". They said if it happens again one of the staff members that lives near me will come around and collect the car and leave me a loaner. So pretty good service. AND.... got they to change the wipers. They did not query it and said they'd change them straight away so maybe they know the factory ones are hopeless. They are Bosch but look slimmer profile to the originals. Hasn't rained yet but using the washer fluid they seem better. So if you get the warning in the middle of nowhere, maybe try and disconnect the battery and reconnect - not sure what the gateway reboot is though. I asked if the 12V batteries needed replacing often and they said "no".