Sorry for the late reply - I've still not sorted anything more than trying to wire up my spare router (Linksys WAG160N) and it sort of works I guess!
It's plugged into my top room powerline adapter and then the PC plugged in to one of the ethernet ports.
The issue is I've not changed the SSID and so it's showing up as a separate network rather than just an "extension" of the main TP Link one! - Is it literally just a case of calling it the same name as the main TP Link network and making sure the channels are suitably apart? (I assume then it will show as a single network rather than two??)
OK, the wireless on my TP Link has both 2.4GHz and 5GHz and both channels are set to "Auto" (as is the "channel width) - do I need to make sure these aren't overlapping too? (and does it matter what numbers I select - are some better than others??)
Also, I assume I need to assign the same WiFi password to both?