Thanks for the replies guys. I disabled PhysX GPU acceleration but unfortunately still only two cards appear in NVIDIA Control Panel System Information and Windows Device manager:
As you can see in the above screenshot I also tried installing the latest drivers from the nVidia website, but that didn't seem to solve the problem..
What else can I try to investigate the problem? Is there anything else I can try with drivers/configuration? How could I go about establishing if it's down to a faulty motherboard/graphics card?