I've now got this working. The short answer is not to use the pairing buttons. There's apparently a management tool, tpPLC, that you can use to manage your powerline network. The software is...

I've had a powerline network configured for a few years already using the TL-PA4010 and the TL-WPA4220. I wanted to try and add some additional ethernet connections so I bought the TL-PA4010P KIT...