There is tones of free software on the market which monitors your bandwidth!
And there is a big difference between going over your bandwidth by 5-10GB or even 20 at max! Then there are people like you who take the pi** with 150-300GB a month!
and claim you don't know because NTL don't provide a monitoring service, well you do know or you wouldn't have posted this thread!
if the cap is no good then downgrade to a service that offers what you need and if one is not available then go elsewhere!
Ohhh and I play loads of online games and I watch loads of video streams and I have never gone over 75GB ever, I even download the odd music track here and there! not to mention my PC is on 24/7 as I have my own site!
if your doing an easy 150GB a month then you want to get rid of your P2P...
(Or use it more wisely)
Edit: I also have my PS2 connected and I use VOIP, Although I'm against caps I would rather have them and have a good stable connection than not have them and have nothing but trouble! (Providing the cap is a fair one and not a stupid 1-3GB like some have)