@FluffFreeman 

Let me summarize what you have, for better understanding, then I'll give my recommendations.

You have Virgin cable router hub3, I googled and found its broadband speed is under 400Mbit. Meaning, you have Internet connection with speed under 400Mbit. This is important to know when planning for Mesh system or asking for advice.

Your Mesh consists of three Deco units: Main Deco wired to router, two Satellite Decos connected by WiFi to Main Deco.

You run your Deco nodes in AP mode.

All your Deco units are M5.

Did I get it right? 

If yes, you have good Mesh setup with proper Deco hardware. It can support your Internet connection speed, it is not underpowered and not an expensive overkill. It is just right.

This is what I would suggest:

A1. If buying additional nodes for Mesh with good setup like yours, buy same type of hardware. It will be M5 in your case. 

A2. Cable connection will always beat WiFi connection for Satellite Deco. Let me emphasize: ALWAYS.

Same goes for all the way Ethernet cable from Deco to router vs. cable from your new future Deco to the Satellite Deco - Ethernet all way is better. 

Meaning - if you can, run cable from your new Deco all the way to router. You won't regret putting efforts in it.

Also, note: if you have free port on Main Deco, you can run cable to Main Deco, if you have free port on router - you can run cable to router. Whatever is more convenient for you. Both will work equally well, because Main Deco is connected by the cable to router - you'll get Ethernet connection all the way.

Finally, recommendation on your Deco mesh configuration. If you have Main Deco and Virgin cable router close to each other, I would suggest disabling WiFi on cable router. Having two strong WiFi network signals near each other could create interference and degrade WiFi performance around them. You are also not getting full benefit of Mesh if you run two WiFi networks in your house.

If you are OK with it, its up to you of course. Not a big deal.

@Alexandre. 

That great. Yes you interpeted correctly. Only 110Mbps at the moment but I am trying to make future proof while the ceiling is down.

It sounds like a star connection is the way to go and take everything back to the router. Many thanks.

I slight shame I am not able to go to router mode for the Deco as there are far more options in this configuration.

Many thanks.