Phew, finally. I've got to the point where I have been able to shut down the higher-level functions of all three of my Bytemark VMs, leaving them just running init, udev, syslog and sshd. All end-user services are off! I'm leaving them quiescent overnight to allow their final backup to happen, then it'll be time to fire off an email telling them I'm finished and they can shut them down and destroy the disk images. The web control panel doesn't seem to let me in any more so if I actually shut these VMs down entirely I wouldn't be able to fire them up again if I found I urgently needed to for some reason, which is why I'm doing it this way. But wow, having that one for 16 years meant there was a lot of integration into my other systems, which I've hopefully fully unpicked now.