Your TP-LINK kit is in a different powerline network as powerlinks, right?
How about status of all lights on TL-PA4010 and TL-WPA4220? Did you pair them and make them communicate with each other?

The lights were blinking as follows -

adapter ;
power -solid
powerline solid
ethrnet - blinking

extender
power- solid
powerline -blinking
ethernet - off

which looks as it as it should be

However - when the links are connected the PC will not connect to the internet.
if I disconnect the adapter from the router and restart the router internet connection returns.
I presume that I have generated a conflict somehow.

A related question.
I already use a pair of adapters (D-link Talkttalk not TP link) that are used to connect to a freeview box in the next room. The freeview box is connected to the 2nd adapter via a cable.
I was trying to set up the TP link pair by plugging the adapter via an ethernet cable to another LAN port in the router and then situate the extender in a different room.
Are the 2 sets of adapters likely to interfere? Do I actually need the TP link adapter or will the TP link extender receive from the D-Link adapter?

There are four lights on extender. How about Wi-Fi light? Can you try to connect to extender by cable to see if it works? If it is the same, try to reset them and rename them using utilty.
http://www.tp-link.com/en/article/?faqid=258
I'm not sure if TP-LINK can join in your exsiting D-LINK kit but your current connection should not case any interference between TP-LINK and D_LINK.
They work seperately in two different private powerline networks.