OK, so Macrium Reflect creates a file called
backup_running in the target directory while a backup is actually running, and deletes it once it's finished. Except if the system undergoes a spontaneous reboot, like it does sometimes, when one is having one's tea. And then you can't re-run the backup, because it's still there, and you can't delete because it claims to be locked by another process even if you've restarted the box in so-called "Safe Mode". Any guesses as to how to nuke the damn' thing? It's on a NAS if that makes any difference.
Edit: Shut down, checked backup directory from another PC, offending file has buggered off. Not there when being-backed-up-PC restarted either