@IanHarker How did you test / what did you use to test the Deco unit's speed?
The ports on Deco M5 are 1Gb, but keep in mind that wireless on 5GHz, the speed is 867mbps max.

So if you were wirelessly connected and performed a speedtest it is normal you wouldn't get 1Gbps.

@DenniSundaY hi using the deco app to perform a speedtest under more in router settings called test internet speed, I presume this should be the port connected to the router in modem mode, all test are wired with cat6 cable

@IanHarker I forgot about the build in Speedtest, and that indeed tests your connection. My bad!

You already mentioned that the router (not deco) itself does give 1Gbps, so that's strange indeed. I can't find more info about it because not so many people have gigabit internet connections to their homes. 
Are your deco's set up as router or access point? When in AP mode, some features are disabled which may improve performance.

@DenniSundaY Hi they are setup as router, i think the speedtest in the deco is being CPU bound in the deco itself as i can get some cat6 laptops to show approx 900mb from within the intranet, which is going through the deco m5, so i think the deco is showing the wrong results

I know that speedtest can give false info in a browser if your cpu isoor, so i think speedtest need to change their software or tp-link need to use something else like samknows or fast.com, as time goes on more and more will be getting gigabtye networks