Well, I've spent a week playing with this and have concluded that the hardware and local network config are fine - the only conclusion I can come to is that it's a bug in limewire.

With one PC, one router and no NAT or firewall in the way, LimeWire appears to act is if it's behind a firewall and insists on pushing content to people downloading files from it. Something is confusing the LimeWire mechanism that determines if sockets on the local PC are visible to the outside world.

Happy to provide details to anyone who fancies debugging, but sadly can progress no further myself. In the meantime, I'll be running as a leech.