I've been keeping an eye on Wildfire deals and think this is the best at the moment. By redemption you get 12 months free.
It is with buymobilephones and on an 18 month T-mobile contract. This includes 600 minutes, 500 texts and unlimited internet.
Cost = £30 x 18 =£540
minus 12 month free = £180
minus £30 cashback = £150 = £8.33pm
This is a redemption deal so only go for this if you are organised enough to send them off. I personally have done redeption deals with lots of comapnies over a number of years and had no problems. My OH currently has one with buymobilephones and has had the first 3 of 4 cashbacks with no problem. However this is not guaranteed!
Also worth noting is that T-Mobile are merging with Orange so network coverage will soon improve. (As part of the deal they are also sharing networks with Three).