Just a few naive questions to the STW massive
Once my contract is up on the i-phone can I use any sims card on any network?
If so whats the best option available?
Also what's the going rate for a 3G 16GB if I choose to sell – seen them going for £249 on ebay.
Also if I do sell how do I wipe my personal data but retain the apps for the future purchaser?
cheers 🙂

You should just get an unlock code from O2 – may take a couple of weeks to get to you though. As far as best option goes, that depends wholly on how much you use your phone,

Also what's the going rate for a 3G 16GB if I choose to sell – seen them going for £249 on ebay.

I'm not sure but that sounds a little optimistic to me for a 3G. You should just be able to do a factory reset which I 'think' removes all but the standard apps (though I'm not sure about the latter).

You don't have to wait for the contract to end to get the phone unlocked, get it unlocked through this page, didn't take the suggested 2 weeks for me,

https://www.o2.co.uk/apps/help/help?qid=1&q1=2&route=unlocking&case=Handset%20Unlocking%20Form