@WHE77ON 

You said you've got five Deco M4, but your diagram shows just four. Keeping fifth one as a spare, perhaps? If yes, it may come handy.

Satellite Deco units prefer to connect to Main Deco (Primary Deco), that would explain why your Office Deco tries connecting to Living Room Deco. In your case, it should not be a big deal, as you plan to wire Office Deco.

For your diagram to work without modifications, you will need to change Deco from Router mode to Access Point mode. See How to set up Deco to work in Access Point mode. When you do that, you'll be back to IP range 192.168.10.XXX for all your home network devices. Your Office Deco will use Ethernet backhaul when you wire it.

In Access Point mode, your Main Deco must still be wired to the home network, it is a requirement, but it can be wired anywhere there is an available Ethernet port providing connectivity to your home network. 

Also, note that Deco does not extend your ISP Router/Modem WiFi. Deco runs its own WiFi network. For that reason, it is recommended to turn off WiFi in ISP Router/Modem. If you need WiFi coverage near it but signal from other Deco is weak, connect your fifth M4 by short Ethernet cable directly to one of ISP Router/Modem Ethernet ports. You can do that after your Deco is configured to run in Access Point.

  @Alexandre. Thanks for a quick reply.

You are correct, I have kept the 5th Deco as a spare rather than selling it.

In terms of the router/modem, I have already switched the wireless off on this (I should have said), and created a WiFi netowrk with the same SSID and PW to simplify. 

I will try your suggestion tonight and come back to you. 

Thanks for the help 

  @Alexandre. Hi Alexandre, that has done the trick perfectly. Thanks.