Yes I know how stupid I've been and will severely chastise myself once this has been resolved.
Just to be clear, I've done a robocopy using the /mir switch and files not present in the source have been deleted from the destination - this was not desired.
I have an exact list of files that have gone, but I have no idea how to get them back.
Unfortunately the source drive was a Linksys EFG80 and the target was a Netgear Stora so I cannot remove the drive(s) and use common recovery utilities as both the units are using some unknown form of RAID.
I would restore from backup, and I believe there is a full backup but due to staffing issues I cannot get the credentials until next week but I could do with restoring this asap before users get in tomorrow.
Most of the NAS devices are based on Linux, and the Raid is normally created via MDADM. What you could try is to attach the disks to a Linux machine and see if you can assemble the raid and mount it (read only of course). Then you can apply standard recovery procedures.
This works in many cases.
You could also check if there are any big forums around these Netgear NAS systems - there are often sticky threads dealing with recovery scenarios
I once dealt with a Thecus NAS that didn't actually delete the files when accessing it via SMB/Samba, but used the Samba VFS recycle bin in form of a hidden directory. To really delete these files, you had to empty that directory.
Maybe this is similar on the Netgear, so check for hidden directories in the root directory of the share.
You could try Free Undelete or Recuva. YMMV.