they also do a 30 day contract too as well as the 12 month one. They charge a connection fee for it but its worth bearing in mind if you want to get out in 6 months time or whatever.
I've been with O2 for about 3 years now and have phone with them too, so costs £20 for broadband and phone plus I pay extra £5 for add ons, ie CLI etc. I switched to the 30 day one at Xmas as I was toying with the idea of going to BT Infinity this year as my exchange/cabinet was enabled quite early in the roll out.
Not had much problem with them, also when your 12 month contract is up they have always offered me 3 months free as an incentive to take out another 12 month contract so last 2 years I got 12 months for price of 9.