Follow up. Boot seems to have fixed itself. So either the cable was sticky or the lock wasn't getting juice to close. Is this a symptom similar to a problem I had with another 5 Series where the electrical cables were getting frayed at the point between the rear lift gate and the body? No idea.
So it may be wear and tare on one of the locking parts, which would be a pain as looking at the ETK there seems to be a few options. So will see how we go and report back. If the problem comes back and no easy solution is at hand then I might resort to an Exorcist first, (would have to be cheaper than the dealer).
