Huawei G300 Ascend - on PAYG.
At the moment I still have it on Vodafone (£100) for the phone. £10 a month top up gives me 500mb data and 300 texts (other PAYG options are available, LOL), though I will probably change to a Giff-Gaff sim and get all you can eat data for the same amount (if I choose to put that much credit on every month) - saying that, this is my first smartphone so I am still working out how much data I use per month.
As for the phone itself, it rates very highly in reviews of budget smartphones but I guess to an extent it depends what you want from a phone, though I have yet to see what I am not getting that other people forking out £30 plus per month are getting. At worst, if I use all my data allowance, I would then have to pay £1 per day for data access - which at most would be £31 per month (less whatever time it took to use up that first part of my allowance).