@Ian-R 

Thanks TP-Link for your response.

I am now quite perplexed.It seems that the disabling of the 2.4 GHz may have been irrelevant.

First, I checked my PC's Ethernet adapter and it shows a speed of 1 GBps (Realtek PCIe GBE Family Controller).

I have today run a myriad tests including:

• PC directly connected to the broadband access point
• PC connected to main Deco with other Decos switched off and no other WiFi devices
• PC connected to main Deco and other Decos switched o
• And all with various wifi devices either switched off and then switched on progressively.

In each case I switched 2.4GHz on and off.

Mysteriously, I can no longer reproduce the lower speeds that I previously reported. I have no idea why. Given that the speed of the broadband services bounces around quite a lot (the slowest the Deco reported was 41.0, fastest 62.3), it is difficult to conclude significant loss of performance through the Decos (PC via Ethernet varied from 31.3 to 50.9, WiFi devices varied from 33.3 to 56.8). Whether 2.4 GHz is enabled appears to make no difference.

So on the one hand I am happy that I am not seeing slow speeds. On the other hand, I cannot explain why prior to yesterday I NEVER saw a download speed on any device above 20 Mbps, and I've had the Deco boxes for a couple of years. I am wondering if there was a dodgy cable or connector and unplugging and replugging has resolved some issue?

If and when I see slow speeds again, I will quickly check the speed that teh Deco is seeing off the broadband service, which clearly varies a great deal.

Ian.