OK. I did this (clean card, flash image again, and boot, just leaving it to install) (but couldn’t resist leaving the monitor plugged in so I could see what was happening). Watching the DHCP on the router it did get an IPv4 and IPv6 address, so the ethernet hardware does work.
The install completed OK with the net working this time. That suggests that there is some sort of bug so that if the ‘first install’ script is run first with no internet, and then run again with access, something breaks in the networking config so you end up with neither the normal networking nor the local hotspot running.
Interestingly, whilst the install appears to work, the other issue I noticed (with the missing icon fonts in firefox: Deb package 4.1.1 not displaying icons in web interface on locahost with firefox - almost unusable) is now happening on this install even with a remote browser. I’ll try to narrow that down further in that thread.
I guess this will have to be filed at one-off weirdness unless I can be bothered to reproduce with a new install to show that it is repeatable. Still, I’m glad that my ethernet does actually work and I now have what appears to be a viable homeserver. Now I can do what I actually started this all for, which is monitor one-wire temps.