Hmm, well that was moderately informative and interesting.
Following instructions from https://itectec.com/ubuntu/ubuntu-how-to-enable-hibernate-option-in-ubuntu-20-04/ I “told” Grub about where my swap partition was, updated Grub, then did a commandline
sudo systemctl hibernate
which shut off my mouse pointer, keyboard interface (CapsLock flashes no lights) and started the power and HDD access lights flashing away. Then after a couple of minutes, the system closed down.
So far, so good.
Re-apply power though and Grub comes back up, defaults to Ubuntu, restores from the swap partition (well, I assume from there ; that’s what I changed) to my desktop, the programs that were running …
… and stops. No keyboard handler, no CapsLock response, no mouse response.
It’s progress, of a sort. Not really useful to me - doing a shutdown and re-boot is quicker. But a sort of progress.
Any ideas where to go further?
Oh, HDD speed? dd-ing, I get about 44MB/s, which is faster than any of the USB buses on this box, so must be on one of the HP’s internal buses. Meh - good enough. I get faster performance off the Ubuntu on that bus than I do on Windows using the internal hard drive on the laptop.