The wife looked at the pictures
here and confirmed it was the same error.
Someone from BMW emergency services came to the house and, as expected, the diagnosis was a low battery level (as given to a few other people on this thread). The car hasn't been left for any period of more than a day recently and did a 50 mile round trip on Sunday so I don't really buy that as the cause.
Do you think it's better to wait for a repeat occurrence and take a picture of the error or to push for more action now? In two minds....