Well that was simple.
I ordered the Buffalo Nfiniti Wireless-N Dual Band Ethernet Converter from Amazon on the 1st January. I used the Super Saver Delivery option, I was in no hurry and why pay extra for postage (especially over New Year). Really surprised to see the parcel arrive this morning! Excellent service Amazon, well impressed.
It’s smaller than the picture makes it look.
Very easy to configure, I plugged it into the power then plugged the supplied ethernet cable into my iMac, reconfigured the ethernet settings in System Preferences, turned off the Airport on the iMac. Then went to a browser, typed in the 18.104.22.168 address, entered the username and password. Added my Airport network details. Very pleased to see that my 802.11n 5GHz network was recognised, remembered to use WPA2-AES (as that is what the Airport Extreme uses). Click configure,update. Job done!
The reason the Buffalo device appeals, is that it comes with four ethernet ports, which means I can connect four devices to the network.
Now to test it out under the TV!