Success! Thanks for being a great community and helping me work this out.
The root_squash was the problem and I have now updated NFS to use no_root_squash.
johnny:/c/nfs# cat /etc/exports
“/nfs” *(no_root_squash,insecure,insecure_locks,rw,async)
“/work” *(insecure,insecure_locks,rw,async)
dietpi-backup now runs fine and all files are now with user=root and group=root.
I can now continue to back-up to the cloud and my RPi4 config is safer than it was.