cheers for the responces guys, to answer the above question as you all probably know if your iphone is visably seen to be jail broken then obviously any repairs for free under the warrenty would be null & void.
However once restored the only way of checking would be an in depth look on the engineers part digging around in the delete logs, but a) apple engineers haven't got enough time to be checking every iphone that comes into repair in that much detail as youd imagine.
So as long as you dont come in to an O2 store with a wicked theme and cydia on the home screen you should be fine to send it away for repair.
I mean if completly by chance they realise it had been jail broken one time or another the worst thing that can happen is that they give you a quote to pay them to repair it or you dont pay anything and have the phone sent back and then get it repaired with a 3rd party company such as fone doctor.
*strange question i know, but on all the pics of jail broken iphones i never see the official app store icon only the cydia icon, does the app store work on jail broken handsets?