<<< should I disable the onboard LAN on the motherboard as I suspect there might be a conflict between this and the wifi card. >>>
That shouldn't be necessary and there shouldn't be a conflict. There are situations where it is necessary to have two network adapters in use simultaneously..
I can actually have a "wired" and a "wireless" connection at the same time between my router and PC without it causing a problem.
<<< I am unable to get an internet connection on the new one with the BT card. >>>
Have you actually "Connected" to the wireless network? In other words, have you run the wireless adapter's software, scanned for "Available Wireless Networks", found and highlighted the one that matches your router's SSID (network name) and selected the option to "Connect" to it? Once "connected", the little icon will get its "halo" to indicate it is connected to that network and it should then get its correct IP address by DHCP from the router.