Hello  @Kev10000 

New power line usually could be plug& play and the House Led (Powerline  Led) will be ON on the 2 units, if not, you could try to pair manually or Utility: 

1. Try to pair manually: please  put 2 powerline unit in the wall sockets, in the same room, and pair them again :
Paring Note:  press Pair button only for 1 sec, Power Leds should be flashing on both powerline units in pairing process. ( if the Power Led does not blink, press it for 1 sec again)
When the Powerline Led or House Leds are on, the pairing process is done

FAQ: How to secure (pair) the Powerline Network?
-  Connect main powerline unit to router via Ethernet cable, and put powerline extender in another room 

2. Try to pair using tpPLC Utility 

Please factory reset your powerline unit by tpPLC Utility one by one:
https://www.tp-link.com/support/faq/1612/   

Refer to this instruction to pair the powerline units/ add an extra powerline unit by Powerline Key: 
https://www.tp-link.com/support/faq/1615/

Can I know if the power Led blink after press the pair button for 1 sec? 

Factory reset TL-WA4220 requires keep press the reset hole from the button until most Led flash once (new version reset about 1 sec, older version about 10secs)