if you are in the first 14 days of install you can cancel because you are still in the cooling off period. Then sign up online to get the offer....
http://www.home.ntl.com/icat/broadband
all the prices there are showing the 3months half price at the moment
However, the offers are to actract new customers, so it makes business sense to only offer them to new subscribers- you will find this is a feature of every company- the mobile phone companies do it really often.
ntl always have some other or other running, i think the last one was 2 months free, so surely when you signed up you got some kind of special offer....?