Log onto the Sky hub:
Disable DHCP
Change the IP address to one within the range of your BT Hub (If your BT Hub is 192.168.1.1, set the Sky Hub to 192.168.1.2).
Set the same SSID (Network Name) as your BT Hub.
Set the WPA Password to the same as your BT Hub.
Change the Wireless Channel so that it's different on each router (If the BT one is on Channel 1, Set the Sky Hub to Channel 11).
This should create one big network and allow use to use the ports on each router as s wired switch. Your wireless devices will also "roam" between each router depending on the strongest signal.