Seamless mesh has nothing to do with signal strength, it just means you can move around with a mobile device and it will switch between WiFi signals without a slight interruption.
Black, complete WiFi discs will accomplish that.
If you have the white disks then the only way you can achieve one seamless mesh with one SSID is to disable the Wi-Fi on the hub
https://www.bt.com/broadband/complete-wifi
So you should have been supplied with at least one black disc as part of your package.
THANK YOU for all your help guys - now sorted.
I've rang BT and they are going to send me a black disk so I can have Complete Wi-fi (which I'm paying for).
My confusion was not understanding the difference between Whole Home and Complete Wi-Fi. Now I do.
Thanks again
Emyr
If you need them, I think you can request up to three.