Hi,
After reading the many many posts in here I thought I better post something up to try and help.
The company that you have sent the PC to for repair are talking rubbish the fan cable shown in the picture is standard practice. The only way the CPU would overheat would be of the cooler completely failed. The other possible cause would be if the cooler and blown or possibly leaked and taken the card out too.
Just to clarify this is not a build fault definitely not after 18 months and just a standard hardware failure.
I know this thread is going to go on for a while so I will just let you know if you post in the support thread I will get the computer back for repair and replace anything in warranty. We will discuss details and what is going to happen in there and we will sort out the PC for you.
You can continue to chat with the guys in here no worries but I am willing to speak directly on the support forums if the guys here cannot help.