Update. 

I have turned off all three slave Decos (two M9+ and one M5) and the network seems stable with all ethernet attached devices working and all wifi devices in range of the master Deco with a stable connection. 

However, this means two things:

1) I have very limited Wifi range as only 1 of 4 Decos is running. The whole point of having ethernet backhaul and 4 Decos was to ensure good coverage through the building

2) If everything else is on an working when the Deco slaves are off, that would indicate there is an issue between the Master Deco and the Deco slaves.

Perhaps there is something I need to configure on the ethernet switches, in particular the Netgear switch?

Thanks

Andrew

Update

I have gone through the TP Link/Deco advice found regarding the switches on the network as follows:

My setup is a "Star Network"

1) The Master Deco is the NAT router on my network. The Virgun Media box and Apple Time Machine are in bridge mode (no routing)

2) The Decos were all set up on the same network and show up in the app, even when disconnected/off

3) See answer above. No other DHCP servers.

4) The Netgear switch and the Apple Time Machine are both on their own IP addresses, not the same IP as the Deco.

Netgear switch 192.168.68.105 and

Apple Time Machine 192.168.68.102

All devices have been allocated IP addresses in the range used by the master Deco.