I've unplugged the tv card from the USB hub and plugged it directly into the USB port on the back of the PC and I can now get all channels working (instead of just the one), and without having to have the OEM s/ware running simultaneously.
I still get the "flyscreen" effect on some channels, and HD channels are perfect, although slightly jerky, so still some playing around with the decoder options...
So, if someone else has this problem, they could try plugging the tuner card directly into the mobo usb port.