The cable goes to a nearby unmanaged 100bT switch, its never given me problems before, but I ran :-
netstack eth0 debug (is eth0 the correct name for the onboard network interface?) and got a whole lot of debug out of it, including :-

iface_init: can't get interface linkinfo
Closing Sana2 device DEVS:Networks/via_rhinepci.device (unit: 0).
iface_init: failed

What confuses me is in MOSSYS:S/network-startup it just calls netstack with no device parameters which results in no debug output.
Will keep tinkering...