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Try taking it to the Apple store? if its still in warranty even if its not worth a try instead of the headache its giving you
 
Hi - again the iphone has a custom back plate - not sure why the original owner decided to do this, looks awful IMO, it is like a transparent back plate - you can see the iphone's internals, battery etc.. I am quite sure this means the warranty is void, even eliminating the option for me to pay to have it replaced (which I probably would tbh). It is actually within the original 12 month period - until Feb 2013 actually.
 
Hi - again the iphone has a custom back plate - not sure why the original owner decided to do this, looks awful IMO, it is like a transparent back plate - you can see the iphone's internals, battery etc.. I am quite sure this means the warranty is void, even eliminating the option for me to pay to have it replaced (which I probably would tbh). It is actually within the original 12 month period - until Feb 2013 actually.

I don't think so.

Just taking off the back does nothing... I took my iPhone back off out of curiosity, there is a void if removed tab over the battery though...

You could just buy a new backplate and swap it
 
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