Well that seemed to work. The migration process got wedged at the very last step, updating the Hyper Backup package but I eventually gave up and tried logging in and it all seems fine. All accounts and permissions restored, even AssetUPNP still works which surprised me greatly.
I will definitely be buying another natty little WD Elements backup drive.
https://www.argos.co.uk/product/8239842
They’re so much less clunky than what I’ve been using until now - which I guess is the technology of 15 years ago.