@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.