Quidco are currently offering a £70 cashback for the o2 home broadband service. If you are an o2 mobile customer, you can get the standard package for 7.34 a month. Aftter Quidco it works out at just over 1.50 a month for a 12 month contract! (Thanks Moob)
Also there is 3 months free on the two other packages, if you require faster speeds.
STANDARD 12 x 7.34 = 88.08 - 70 = 18.08 p/a divde by 12 = £1.506666 per month
PREMIUM 9 x 9.79 = 88.11 (3 months free) - 70 = 18.11 p/a divide by 12 = £1.5096 per month
(sorry for going all Vorderman)
So you're better off getting the Premium package, as you get faster speeds for less than 3p extra than the standard package over the course of a 12 month contract.