Actually here is some info for you that you may want to pass on to Samsung (and by proxy, Three).
There is an issue called SDS (Sudden Death Syndrome). There is also an issue called Half SDS.
SDS is that one day (usually after being charge over night), the phone is dead and will never power on again. Black screen, no LED, Nothing you can do. Dead as a Dodo
Half SDS is similar but can boot into Download mode.
The trigger for both is the same. On GT-i9300's with the VTU00M eMMC chip, Firmware revision 0xF1 - there is a bug in the wear leveling of this firmware (assumed when reaching a block where it cannot write data and continues to try) which causes a catastrophic failure of the eMMC (Embedded Multi Media Card / Internal memory). The failure wipes out the file system on the eMMC.
This causes an issue in download mode:
PRODUCT NAME: Wiped Blank
CUSTOM BINARY DOWNLOAD: No
CURRENT BINARY: Samsung Official
SYSTEM STATUS: Custom
Now this last one in download mode, is how they check if you are rooted / custom or not. The problem is that the half SDS Sets this to Custom by default. So one of the sysmptoms of this failure is that custom status is flagged.
Some Samsung repair centres know this. Others do not.
The lucky thing is all Samsung need to do is a google search as this is well documented on XDA developers.
Here is a link:
http://forum.xda-developers.com/showthread.php?p=36873352#post36873352
developers have been looking at this issue for some time now. We have seen a few people have their warranty denied for this reason but Samsung need to suck it up and repair the phone for you. The problem is their buggy firmware on teh eMMC is making your phone FALSELY report a custom status.
I am happy for you to link samsung to this post. And 3 for that matter.
I sincerely hope this helps. Both 3 and Samsung need to wise up on this issue.