Never mind - dug out an old USB 1GbE adapter (also Realtek, as it happens) and a network connection was achieved.
Created the log batch from the Logfiles share - attached here log-2023-12-09-15.40.59.zip (300KB)
The onboard 2.5GbE ethernet devices (eth0 & eth1) both seem to be detected and started without errors that I can see, but for some reason they don't show up in LE/Kodi.
My previously working device on the older nightly builds is eth1.
The temporary 1GbE USB adapter is eth2.
Can anyone see why eth1 isn't working ?