Orange Dolphin 25 / 24 months
Get 7 months free by redemption, and also possibly £50 Quidco. Works out at £15.63 a month (total = £375) , or £17.71 without Quidco (total = £425)
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.