The new system was relatively low value (some setup effort, but nothing hard to replace), so I decided to mess with that instead.
I booted from USB (using the mint install system), and did the following (where `sdb5 is the encrypted partition that houses the lvm data for my new system)
cryptsetup luksOpen /dev/sdb5 newdisk
I'm not sure that those scans are actually necessary, but i did them, and then I looked around with
lvdisplay. All looks fine.
I tried re-booting at this point, which didn't work. Grub couldn't find the disk, and complained with a message like this one.
I booted back into the installer, and fixed up the references to
The system boots, and I can now mount the old drive using the GUI tools (
nemo), albeit it's a tad inelegant, with an error displayed from the attempt to mount the encrypted partition, after which the unecrypted one is available to mount.
So, it seems OK. I've
mint--vg in /etc and /boot, and found nothing else to fix up. I'll add more info here if I come across anything else that needs fixing up.