@DBurn 

Hello,

May I know your network topology？

Did you mean before you plug the main one to the TalkTalk Youview Box, the powerline light of the other one of TPLINK will turn to red then off?

If you unplug the Youview box, the powerline light will come back? At this time, if you plug it again, did the powerline product will go off again?

How many powerline products did you have? How did you pair them if the powerline light will turn red before the main one connected to the TalkTalk?

Here is an instruction you can refer to pair them:

https://www.tp-link.com/en/support/faq/1089/

When did you purchase it and when did this issue happen?

Please try to change the different power socket to plug them. 

By the way, what is the model number and hardware version of the powerline product, you can check it at the bottom label of teh product.

Have a nice day!