You just freaked me out for a second there when I couldnt see it either, but its the simplicity tariff for smart phones you want:
http://shop.o2.co.uk/tariffs/simplicity/smartphone/12_months £15 a month for 12 months gives you 300 mins a month (more than enough for me) unlimited texts and unlimited data.
Will be nice in 12 months time to hopefully still be using the desire and just paying £15 a monthly, rather than being tied into another 12/18 month contract again (I am assuming once contracts lapse it just goes over to a monthly charge, rather than a new contract)
The one thing I will say against this though, is that buying the phone outright makes me feel its less "protected". On a contract if it went wrong Id have no qualms sending it in to get fixed/replaced. Technically its covered under warranty buying it outright, but its just got a little less peace of mind this way if you get my meaning.