@bill-lancaster Thank you for the advice, I'm a lot clearer now.

Regards

  @bill-lancaster Not there yet though!

The TL-PA4010 in the summer house has both the 'Power' LED and the 'PowerLine' LED on but my wifi network is not available.

Installed a TL-WPA4220 and noticed that there was a network available called TP-Link_590.  Have failed to make a connectionIs to this network, is this possible?

  @bill-lancaster Found the connection details on the back of the unit. My device now logs into it but I realise that this probably won't give me access to my main wifi network.

  @bill-lancaster 

The unit labelled TL-PA4010 is the one that has to be connected via Ethernet cable to your router and the TL-WPA4220 is the one that has the Wi-Fi and therefore has to be plugged into a socket in your summer house.

Once it has been set up like that and also the Powerline LEDs are solid ON on both units the network should be operational and if you connect youself to the "TP-Link_590" Wi-Fi you should have access to the whole network that your router provides. It doesn't matter that the Wi-Fi name is different.

If you feel having a different Wi-Fi name and password in you summer house is inconvenient, then you can change them to be the identical to the one's of the router.

There are two ways to do that (via the configuration interface or by pressing physical buttons, aka "Wi-Fi Clone") and both methods are described in the user's manual of the Powerline adapter, which can be found here. https://www.tp-link.com/en/support/download/tl-wpa4220-kit/

  @bill-lancaster 

According to the user manual (see below) a blinking Power LED indicates the Powerline adapter is trying to pair. If it's not blinking, then it is not in "Pair" mode.

Try pressing the Pair button somewhat longer than one second.

And it's the same for both Powerline adapters to be paired. If the Power LED on the adapter that is in power save mode doesn't start blinking, then it likely means pairing doesn't work in power save mode.

If none of your three TL-PA4010 units is currently active (all in power save), then you will need to find a way to wake at least one up to pair it with the TL-WPA4220. 

  @bill-lancaster Thanks for that.

I moved the WPA4220 to a socket right next to a working PA4010.  The PA4010 had all its LEDs ON and steady.

The WPA4220 has only Power and the lower (WIFI) LEDs ON.

Pressed the PA4010 link button for 2-3 seconds, Power LED starts to blink.

Pressed the WPA4220 link button, nothing happened.

With a pin I pressed the reset button for 10-12 seconds, all LEDs went off then just the top(Power) and bottom(wifi) came on.

I've been messing about with this poor WPA4220 for several days now and it seems to have stopped working.

I can connect my mobile 'phone to it and can see a number of the adaptor's settings.

  @bill-lancaster 

That is very unfortunate.

In case it was just the Pair button that has given up, there is actually another way to add a new Powerline adapter to an existing network by using the so-called "tpPLC Utility" that can be downloaded from TP-Link's website.

Instructions can be found here: https://www.tp-link.com/en/support/faq/1615/

  @bill-lancaster Thabnks woozle

I'm all Linux at home so borrowed a W10 PC and installed the app, added the adaptor, I now have Power, Power Line & WIFI LEDs ON.

Phew!

Thanks for all your help