www.affordablemobiles.co.uk
I use that website when renewing, but just pay attention if its a "cashback" offer as thats just a little bit of extra hassle. If you decide which phone you want you can see what offers they have, and to my knowledge they only offer 24 month contracts. Given the designers build in obsolescence after 2 years I wouldn't be signing up to a 36 month contract.
I would say though that there seems to be a significant increase on the cost of a top end phone since I last renewed. You used to be able to get one for £40ish, and now you're looking at £60ish. I don't think the technological advances justify it personally. I think I'm going to go with last years "new" phone.