Hi, & welcome to the site.
You are not the first to be hit by this issue, & you won't be the last.
Unfortunately, I don't think you can blame ntl (or any ISP for that matter) for someone not properly protecting their PC.
It is quite a common problem, & may be worth a look at the ICSTIS website, as there are actually laws that apply to premioum rate diallers (such as each call must not amount to more than £20.00 or something like that)
Ultimately though, I think you have just learnt a very expensive lesson that most BB users learn-unplug your phone from your PC & make sure your PC is clean of any virii/spyware/diallers that shouldn't be on there.
BMW make cars, but you can't blame then when people don't drive properly & have accidents, & that's pretty much the case for you-ntl supplied your BB, but it was not their fault that your PC had diallers installed on it.
Just my 2c......
http://www.icstis.org.uk/icstis2002/default.asp
Out of interest-what numbers have been dialled:
http://www.icstis.org/icstis2002/default.asp?Node=67
Good luck, & keep us informed.