NTL do have plans to make tools available to monitor your own usage.
Until then, they aren't really enforcing the cap unless your heavy usage is affecting network performance.
I was told by ntl on the phone a while back not to worry at all, it was when there was a 1gb a month cap and i went over by about 8gb.
Scroll to the bottom of this page anyway, says here that they plan on making the tools available by December.
http://www.home.ntl.com/page/usagefaq