Devices connected to the Deco network no matter via WiFi or Ethernet, should be able to talk to each other.
If your wired devices aren't able to talk to wireless devices, check their IP addresses, make sure they are in the same network segment.
If not, then you should consider one of the following:
1, Put your Sky box to bridge modem, let Deco become the main gateway of your network and everything connected to the Deco.
2, Configure the Deco to APP mode.

There's one more thing worth noticing:
For the Deco the primary WiFi and Ethernet are within the same network segment, while the guest network is in another.
If you have enabled Guest WiFi network on the Deco, devices connected to the guest network wont be able to talk to devices connected to the primary WiFi or via Ethernet.
I guess they made it this way for security reasons.

thanks for the reply.

I will be going wired back backhaul (I should have said this in my first post)

My Sky box wont go into bridge mode.

I dont yet own the M5, but this FAQ suggests that it creates its own network http://www.tp-link.com/en/faq-1794.html, I guess if it sends 0.0.0.0/0 to the skybox then I should be good.

I just wanted to be sure before I pull the trigger and bought a set.

I have pulled the trigger and bought a set after reading

http://www.tp-link.com/us/faq-1842.html

I now feel that it will meet my needs if I go for full APP mode.

Good luck!!!