Just thought I'd chip in if I can help. I have analogue cable so it's probably different, but I know some people have accomplished similar things with ntl digital.
I have a hauppauge pvr-usb2 connected to my PC. The cable box is connected to my VCR, and then from my VCR to my pvr-usb2. This allows me to watch and record the same channel currently showing on the cable box.
I then purchased a USB-UIRT (a great device from
http://usbuirt.com) which allows the computer to change the channel on the cable box (as if pressing the buttons on the remote)
I then use GBPVR (free from
http://gbpvr.com) to schedule and record. It automatically changes the channel and records the programs I select in the TV guide.
I've been very pleased with this setup and it works really well. I'm sure it'd be possible to have a similar setup for digital, afterall, the output is the same, and you dont need a built in digital tuner (which I dont believe is even allowed with NTL to decrypt the signal with anything but the NTL box)