(adsbygoogle = window.adsbygoogle || []).push({}); So m iPhone 4 contract ends june/july ish im planning on holding out a couple of months till the 5S or what ever is next for Apple,

But what happens to the sim card once the contract ends ? Can i just slap a £10 top up on it or will it just stop working

You should just be able to put a new sims card in it and use it if you've completed the contract.

once you have completed the contract you can ask them to IMEI unlock the iPhone for you, and they should do it for free. What service provider are you with?