@olivierlambert I think the intricacies of NFS, my seemingly borked NFS permissions were likely the real issue. I rarely have real problems with XO. I tried proxmox but didn't get on with it so switched back.
I'm just a home / homelab user, but always impressed with the community around XCP-NG too and have learnt a huge amount from Lawrence Systems.
Anyway this morning I have a successful backup completed :