SpinSpinSugar
I'm paying monthly. In theory you can cancel at any time, but if you cancel within 12 months they make you refund the installation fee (£50ish), which they are paying for you at the moment.
I agree about the modem, but I still got it from them (you dont have to, btw). I decided that if I was having setup problems it would be simpler if all the kit came from the same place - that way it is their problem, not mine.