Urrrgg what a mare. For the benefit of future me -
When I set up the new server it turned out it couldn’t establish an ssh connection with the old server (and viceversa) as the version running on the old server was so out of date. Upgrading it wasn't really an option due to the risk of making shit worse, and i really didn't want to lose the ability to connect from here as well... So, while i was connected i made some changes to the config (added some newer cipher settings and the like) so that the new server could establish a connection and restarted the SSH process. That worked, and I was able to transfer the mastodon server overnight.
In the meantime, the SSH connection to the old server from here dropped and I discovered yesterday morning that i couldn't reconnect... or rather it would reconnect and then immediately drop the connection. I thought maybe a reboot would help (it didn't)… ssh -v still indicated key exchange problems and the like. Not a problem - all i needed to do was undo the cipher changes that I'd made and...… yeah, now I'd no access to do that.
Normally at this point you'd just walk to the actual server and sort it out, but that's not really an option since the server in question is sitting in a datacentre in Roubaix, which I'm sure is lovely at this time of year but not very practical for getting to in a hurry. To get access back I had to put the server into rescue mode, which is the equivalent of booting a broken machine up from a live CD or whatever. Did this, got SSH access back and... where are my files?
Long story short, I had to figure out what filesystem had to be mounted (/dev/sda5 as it happens) to get access to the hypervisor OS files which for some reason are stored in tar files within tar files (took me about an hour to figure that one out), extracted /etc/ssh/ folder, undid the change i'd made the night before, replaced the tar file, almost modified the wrong files a couple of times in the process, then crossed my fingers and rebooted.
I can now SSH from the old to the new server again which means we're still on for 10pm Saturday night for the big move.