As you're on Vodafone go to:
http://www.moneysupermarket.com/mobile-phones/networks/vodafone/Select the Galaxy S3 from the list, it's currently £19.79 with a 300 minute, "unlimited texts" and 250Mb data. Give them a call and speak to their renewals team or cancellations team and they'll sort you out.
The S3 won the most awards last year, I love mine andfor £20 a month you can't get much better, you're also used to Android coming from an HTC Wildfire S, albeit an older version. The S3 had 16Gb on board and an MicroSD slot.
Alternatively if you want to move away from Android, the HTC 8X is their most powerful Windows Phone 8 device and with the same contract as above from Vodafone. The cheaper HTC 8S seems like a good little phone from what I can see of my colleagues, so I can only imagine the 8X is going to be much better.
The Nokia Lumia 820 is pretty sweet too and looks awesome in red!
http://www.pcpro.co.uk/reviews/smartphones/380482/htc-windows-phone-8xIf you don't mind £27 a month for a shiny new phone, the HTC One is superb! My only gripe is the lack of a Home button which forces a virtual one which does make using it more of a learning curve than say the S3.
http://www.pcpro.co.uk/reviews/smartphones/380965/htc-oneAll that said, the best advice is to go into a shop and play with as many phones as you can to see which you prefer and use the MoneySuperMarket site to get you a good deal!