This is the original Desire model, but its still the cheapest that I can find for people who aren't bothered with having the latest model!
Orange Dolphin 25 / 24 months
Get 11 months free by redemption, and also possibly £50 Quidco. Works out at £11.46 a month (total = £275) , or £13.54 without Quidco (total = £325)
mobiles.co.uk is owned by Carphone Warehouse so they are a reputable company, and all their phones come unlocked. Their cashback procedure seems really easy, you can send ur bills away by email and within a 60 day timeframe.