Hi Kevin,

Thanks for your reply.

As mentioned the Decos are in Access Point mode. I can get internet from all of them if I use wifi backhaul only. I would like to use ethernet backhaul, but when I try this (as per topology in attached picture below) I get IP configuration failure messages, icon is solid red on main deco and flashing orange on other two.

Removing either ethernet cable seems to resolve the issue (but is wifi backhaul).

To answer your questions:

1 for the topology using ethernet backhaul; solid red (main), flashing orange (other deco)

2 see picture below, with this topology I cannot get internet from any node.

3 When I tried to connect the third deco via powerline adaptor to the router as well (via tp link adaptors which I have been using without issue previously) I found that this caused the virgin router to become unresponsive (no internet anywhere, and unable to use web browser to check status of network on 192.168.0.1 until it was manually rebooted).

Plese let me know if you can think of any suggestions to get the ethernet backhaul working for my set-up.

Thanks.

Topology mistake, you need connect 2nd Deco into 1st to work properly.

In AP mode you need all DECOS at same network behind main Deco.

This is not a topology mistake; you are wrong.  This configuration is specifically allowed for in https://www.tp-link.com/uk/support/faq/2399/

I have exactly the same problem and there seems to be some sort of complex incompatibility between the Virgin Superhub 3.0 and mutiple Decos using ethernet backhaul.  

The setup I have worked correctly with a Superhub 2.0 but does not work with a Superhub 3.0.  I am currently in contact with TP Link UK Support.  I have sent them all the info they requested.  I have also sent a Debug log from one of the Decos.  

I think that the ethernet packets that get sent between the Decos are somehow getting interpreted by the Superhub.  The Superghub is a product from Arris.  

I am not sure how I should best get the issue looked at at the right level in TP Link.  I think it will be necessary for you to get hold of a Superhub 3.0 and do your own tests.

Incidentally, I found a strange thing.  The system time on the Decos is not synchronised.  One Deco shows GMT and another shows British Summer Time (an hour apart).

@Kevin_Z  not sure if it's relevant or not anymore, but I had a similar situation with my triple set of deco m5. What solved it for me was... time. I waited for 5-6 minutes after everything crashed, and suddenly the entire network came back to life and all decos got green. It seems the backhaul overwhelmed the router and it took it a couple of minutes to sort this out. This happens every time my router restarts, but eventually everything returns to normal after 10 or so minutes.

Try waiting before giving up...