I installed OMV5 (latest version of course) on to another SSD (to keep OMV4 as a backup) and ran it with the same result - only 5 out of 8 disks found.
I then installed OMV Extras 5 and installed the Promox kernel (following the instructions to reboot and clear the original kernel).
OMV5 saw all 8 disks immediately.
They've been mounted, SMB shares etc configured and everything works fine (with data intact).
Obviously there was an issue with the SAS card and the original kernel.
Thanks again for all of your help
I've also learnt some more about Linux