I picked up another single adapter from Amazon and it arrived quickly and set up was an absolute breeze. Just plug the adapter into a vacant 3 pin socket, plug in the supplied ethernet cable and push the sync button for a couple of seconds on the new adapter and do the same for one of the existing ones. Hey presto all works.
It's still not as fast as full blown ethernet but it has improved the video streaming quality and reliability significantly - plus it's much easier to chuck files around the network.