Ak. Sorry to possibly disappoint you but I'm pretty sure they're a bit dodgy. They phoned me too, and after Googling about them I decided I'd rather not bother.
http://www.certforums.co.uk/forums/l...ead271222.html (pretty long, but lots of different experiences in)
http://www.digitalspy.co.uk/forums/s...d.php?t=981509
I don't think it's a total scam but sounds like you need to be 100% in, if you don't like it and decide to leave it seems they can suddenly say how much money you owe them.