We are now live again with all the VMs - which is a relief.
The above steps from @stormi were great, the one thing I notced was the /etc/stunnel/certs directory didn't exist. I created that manually and everything else seemed happy.
I did have to mess about a bit with the fibre NICs, but I've repeated the previous renaming and that's now running fine.
Thanks very much for all the help - I appreciate it.
@Danp
RTFM is so often the correct answer - this was my error 
I've followed those instructions and done this with the VM shut down and it's completed in seconds.
The migrated machine has boosted up, although with a small delay. I think that's linked to an issue with the NIC. It's not landed on the network but I'll keep digging into what netplan might be doing there.
Thanks very much for your help and for your pateince.
@olivierlambert
Apologies - I wasn't clear at all about that!
I used the script at https://github.com/ronivay/XenOrchestraInstallerUpdater and it does seem to be using systemd
I have found some interesting info so far, but I'm going to make sure I'm fully updated and have a recent log entry to reference.
Thanks very much for yoru help and patience 