Pretty amazing deal!
http://www.quidco.com/o2-home-broadband/ - £70 cash back
http://broadband.o2.co.uk/ - it has 3 months free on Premium (20mb) at the price of £9.79 for an O2 Customer and £14.68 for everyone else.
Which means for an o2 customer a total of £88.11 taking into consideration the quidco cashback you have £18.11 divided by 12 months you get £1.51 a month. Pretty good deal for non o2 customers.
PS: I am not affiliated with o2 etc etc etc just posting a nice deal