logging in as root on a text console doesn't enable ping/ssh etc, so that's a good thing (in so far as I can get on the machine while the problem is present)
firewall is running, default zone is still FedoraWorkstation
the ethernet interface is up, but without anu IPv address doing an "ifup" on it starts it ok (some sort of warning from NetworkManager about using old scripts) and then stuff works
having started it that way, the network does not go down once I've logged out everywher, it stays up untill the machine is shutdown,
looking at the gnome settings gui for netwoking, the wired ethernet is set for "connect automatically"
but "make available for others" wasn't ticked, which seems to be the magic setting I needed ...
this seems to corespond to "all users may connect to this network" within nm-connection-editor
problem solved, but would that setting default to "on" rather than "off" with Server edition?