Andy said:

Was this an upgrade from Ubuntu 22.04 to 24.04 using "do-release-upgrade -d"?

Yes. 

I'll take a guess that Ethernet interface has changed name from eth0 to enX0.

Yes, that is what happened. Editing the yaml file to reflect that and rebooting worked. 

I did notice an update to the 22.04 cloud-init package last night, but looking at another VPS it doesn't look like it's prepared for this change, so anyone else trying this needs to be prepared better than I was. 

Many thanks, particularly for the xen shell making it possible to talk to a VPS that doesn't want to talk to the rest of the net... 

Ian