Thanks for all your help guys, I solved the problem and it was a stinker...
Essentially, my Viera had two NIC's configured for my home network, my LAN and my Guest Wi-Fi network. Some time ago, my switch failed and I replaced it, but during the outage I had configured the Viera to use my Guest Wi-Fi network temporarily, which runs on a different VLAN to my LAN, for obvious reasons. You can't route between the VLAN's as I just want guests to use the internet. I had forgotten this, but re-discovered it when I checked the settings on the Viera before I sent the logs.
Now, because Mac's and PC's handle multi-homed networks differently (Mac in a Unix fashion, Windows in typical Windows fashion), Windows allows and can see hosts that have multi-homed NIC's (Windows allows multiple default gateways, Unix / Mac doesn't). I'm not quite sure if this was the main issue, but it could have been a factor.
However, what I think was happening was the Viera could see the Windows MC through the wired connection, but it could NOT see the Mac MC via the wired connection, as the Guest WI-Fi network was taking precedence for outbound traffic (multicast I guess) from the Viera. Because the Windows MC had been set up previously on the wired connection, the Viera still remembered its MAC address and sent traffic along the wire to it. But it could not see the MAC address of the Mac MC as it had no knowledge of it.
Hope this all makes sense, multi-homing is evil and has caused more network problems in my career than I've had hot dinners!
I'm sure I could crack out Wireshark and prove what was happening, but I'll save that for a rainy day - its fixed and thats the main thing.
Cheers,
Jamms