Hi, just a heads-up really. When my O2 contract recently finished, I called up their cancellations dept and took out a new 18 months sim-only deal.
Upfront cost is £35pm for which you get 600 mins, 1000 text and a bolt on such as free internet. After guaranteed cashback (subject to you sending in at least your last bill), it works out as £5 per month.
I think that this is a wicked deal. I personally purchased an Iphone as it works out much cheaper to buy it up-front and pay £5 per month for the contract.