NIB
My understanding is a waiver you've signed to something you're legally entitled just wouldn't stand up in court. An analogy might be if you (inadvertently) signed a waiver in a shop to the right to get a refund on faulty goods... if the goods were actually faulty then I don't think any such waiver would stand up in court... especially if it was hidden amongst everything else and not made clear to you.
So if you did want to exercise your cooling off period, then I simply would (probably along the lines Denis suggests, except without interest), and if they throw the waiver in your face, then roar IFSRA, Consumer Association, Ombudsman, Solicitors etc etc. in their face. Poor form from NIB I have to say - I thought they had learnt their lessons.
An analogy might