I have just over a month left on my 3g contract and Im paying £45 a month.
I know I would be able to upgrade the phone to a 3GS but would I also be able to change to the iphone simplicity contract as well???
http://forums.overclockers.co.uk/showthread.php?t=18094504 Im talking about that one

If you want to keep your current phone you can get an iphone simplicity tariff. It's £20 a month and you get 1200 texts 600 mins unlimited web and wifi and the voice mail. It's not on the website but if you phone they can swap you over.
I'm pretty sure if you accept an upgrade you will need to sign up for another 18 months.