Well, a quick aside. It turns out Pi #3, being the one I had to move to the only working NAS drive, whilst it booted fine, failed to after an update. But, not the end of the world, it had owncloud on it, and I'd been meaning to switch to nextcloud anyway. So downloaded the latest raspian and started again, Set up the NAS boot before the sdcard even got to the Pi after writing, and all seems well. Nextcloud installed, and working on 13.0.4. Now updating in the running system to 14.0.3 - I think - been backing up a few minutes now, but what the hell.
I wish the tutorial had advised using "sudo chown -Rv" at step 4 (instead of just "-R"), just for a little comfort that the damned thing is doing something.
Nevertheless, if it all goes pear shaped, I know not to start at 9pm on a work night next time.
Oh, for info, I used https://pimylifeup.com/raspberry-pi-nextcloud-server/
Headless PI. OMG, someone cut it's head off. Oh, hang on. it didn't have one to start with.