Not sure if this will help you, but in my setup at home I use my NTL cable line as my TV arial. Not only does this give a crisp picture, but I recieve a few of the freeview channels on a normal TV! (Sky Sports news etc, this is due to the TV having a wider tuning range than most, and the fact some "freeview" channels are broadcast by NTL in an analog format I assume.)
To do this I used a splitter which I bought from Maplin's and an adapter to convert the NTL type co-ax connector into a co-axial suitable for the TV. Together they came to under £10. You are spliting the NTL cable line after it comes out of the wall socket, but before it enters the cable modem/STB. This does cause a reduction in signal strength, but providing you only split the cable once between the wall and STB, you should be fine.
The guy in Maplin told me that this signal would also work with a digital terrestrial tv tuner (ie a freeview box) although I never got to put this to the test, and MattD is no doubt right. I can't really see NTL transmitting all the freeview channels in digital(mpeg2) format, especially when a few are already present in analog format. Still, it makes for a good analog arial though.
As for modifying your freeview box to record NTL only channels, It can't be done. You could always buy a PVR card for your PC

.