After reading through the last few pages of the Mover Tuning main thread it's clear to see that some issues popped up since the Unraid 7.0.0 update.
Normally I don't do software updates very quickly, i've been burned a few times before and was going to let this Unraid update settle down until the next one came along but then came the Big Scary Message in the Unraid newsletter about the Coordinated Vulnerability Disclosure stuff so I decided then to give it a go. I really should listen to myself sometimes though after having had to spend a few hours messing around last night and today fixing my Unraid setup.
Anyway, armed with a bit of new knowledge from this forum I decided that the Mover Tuner stuff has clearly become a bit alpha-level status with everyone poking around in it's code, quickly jamming it back together again along with the various feature requests of some people, so I uninstalled Mover Tuner, closed all the dockers and VMs down, took the array offline and rebooted it. I started everything back up, gave it a few minutes to settle down and let my finger press the Move button on the Unraid main page.
I'm happy to report that the standard mover part is now doing it's thing again as it should, my cache drive has regained back all it's space.
I think I shall just leave it at that, no need for any other add-ons that might go *pop* again one day.
I'd like to thank all the people of this forum who take the time to post helpful messages, they help the picture of the solution(s) load into my slightly messed-up brain