Solved! ...well not really.
Thank you both for helping me through this! I don't really know why it worked, but here's what I did:
First, as I reported above, after booting the DVD in 'try linux' mode, I ran fdisk -lu on the hard disk I've been trying to install to:
root@pc:/home/self# fdisk -ls
Device Start End Sectors Size Type
/dev/sda1 2048 1050623 1048576 512M EFI System
/dev/sda2 1050624 488396799 487346176 232.4G Linux filesystem
Grub rescue> ls produced:
(hd0) (hd0,gpt2) (hd0,gpt1)
where gpt2 was sda2.
This AM I rebooted the DVD in 'try' mode again, and ran gparted on /dev/sda. I deleted both partitions and then executed 'install'. Result:
root@pc:/home/self# fdisk -lu
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 488396799 488394752 232.9G 83 Linux
Restarting, Ubuntu 18.04 booted on /dev/sda and is running as I type this. If anyone can explain why this worked I'd be grateful for the education. But as of now I'm a happy camper.
Incidentally this all started when I upgraded to 20.04, to discover that an app I use a lot (gourmet) isn't supported. Naive me trying to work around that broke 20.04 and here I finally am.
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