Sorry if I'm being rude but why did you sign up to a service fully aware of the price and decide within 2 weeks that it was too expensive? Bit of a change of heart?
Saying that though I'd say you've a fairly good chance of getting some discount. This is after all ntl and while most sane companies would point to your contract and tell you where to go their 30 day money back guarantee leaves them quite nicely open to a bit of extortion
By the way you didn't get the online discount due to the higher costs of selling you the service, through a salesman rather than the much cheaper online way.