Wow thanks for the replies chaps, I honestly thought I was being thick by not being able to find the OS configuration backup that I spotted there being a "Restore" for in omv-first aid.
I have only been using omv for a few short months, and in that time I have had the storage drives (one SSD and one magnetic HD) still showing their contents fine from linux, but just the omv / linux side had gone down.
In one case it was just with omv with the front end vanishing, tried everything I could find on here, and ended up doing a fresh install of just the SD card (My bad, I had not cloned it as it was still a bit of a work in progress and only a couple of weeks old) I think in this case it might have been an SD card issue despite it being a new top spec sandisk type... The two drives I had on that setup were fine, so I just had to fresh install omv and re-configure the system. I just put it down to a corrupted SD.
On the other case I know what caused it, an electrician cut the power to my mad scientist lab without warning, which killed the linux partition, which I recovered with teskdisk from memory and then also had to reinstall omv and re-setup.
It was during this second time that I found the configuration recovery option in omv-firstaid.
I now have two rpi4s running OMV, both also have a cheap rpi specific battery powered uninterruptable power supply on, which has about 4 hours of storage, which pulls talks to a python script on the pi to shutdown the system when the battery gets down to about 25%, which should stop any electricians from ruining my day again (but that also leads onto a post/question I am going to be putting up shortly.)
I also now have a backup strategy happening on a regular basis, with the OS cloned onto a USB stick monthly, and automated backups to one other rpi omv every 48 hours, with 7 days stored, along with a weekly off site backup to a rpi with omv in another building.
Now I just need to refine my electronics and scripting options for some form of UPS on the magnetic hard drives.
Many thanks though, I never expected it to turn into such an interesting thread... I half expected people to shout about "using the search" which I had pretty much exhausted :o)