So I have resorted to a 3 device solution!
VM Superhub 3 - modem mode
Asus N66U - router mode with wifi switched off
BT smart hub - wireless AP mode
This means I am getting excellent wired performance (from the ASUS) and wifi is hitting 300 mbps upstairs thanks to the 802.11ac on the smart hub!
Do you have instructions for getting the smart hub in AP mode with a Superhub3? I know there is a thread or two floating around but I didn't have much luck (admittedly I gave up quickly)