O2 iPhone Simplicity
I found that the cheapest tariff I could get now I've come to end of my iPhone contract is in fact to stay with O2 and get their iPhone simplicity tariff. They don't advertise this but if you ring cancellations and ask for it they will give it to you.
£20 a month for 1200 texts, 600 minutes, and the same unlimited data and openzone wifi you get with the standard iPhone tariff. This seems to be the best iPhone deal I could find whilst keeping unlimited data (or o2's version of unlimited anyway)