Thanks - looking at this again I'm wondering if it would be easier to pull all the data off the drives and start from scratch? I have backed up the content I need and I'm not bothered about losing the backups for my desktop and laptop as I can set the backups up again easily enough.
I've upgraded OMV to 1.0.31 using Putty CLI so had a small amount of practice with that now!
Quite keen not to have to setup all the folder shares side of things again, but OMV is on a USB stick (I know! Going to move it to a 5th HDD once I've sorted this out) so none of the storage drives are used for OMV installation so could be wiped.
Is that a simpler and potentially less time consuming option? If so what order steps do I need to take?