@UrbanB 

Just do a speed test connected directly to a ethernet network cable on the sky router ( not wifi )  ( Speedtest by Ookla - The Global Broadband Speed Test)  this will give you the average speed coming into the house. 

Then compare that to your wifi connections speeds between the Deco M4 and the Sky router mesh wifi. if you have new equipment from Sky its most likely more advanced in the Wifi side that the deco m4 but may not have the coverage/reach around the house compared to the Mesh in the M4 if you have 2 or more units. 

I get 900Mbps coming into the house via ISP modem/router ( ethernet cable and laptop plugged directly )   But my Deco XE75 3 unit system averages out 450-600Mbps around the house ( 3 floors )  over wifi.

  @IntrovertSins 

I've just logged into the Deco app, went into Advanced, Operation mode

Is changing it to Access Point really just clicking on this toggle button? Is it really that simple?

  @UrbanB 

Correct, Turns of the router function so all ip address are given by the connected router/ modem ( sky ) leaves the wifi all working - this is what i am using with my setup. But turns off the parental controls and you may be missing a few other features. 

If you see an option called Smart DHCP in advanced section  i would set that to disable ( has caused a few issues for people and for me )