Dell Precision T5500 no longer booting.

Hi, I'd just like to get some input from you kind folk regarding a T5500 workstation I own.

This workstation has been running absolutely fine until yesterday. The day before I used it most of the day without issue. I shut it down and turned off the mains as I always do for the night and went to bed.

Next morning I switch the mains back on and hit the power switch and computer starts up but then after about 5 seconds it shut down again. Very odd I thought so I tried again a few more times and exact some thing. No beeps no bios screen no nothing. So paying attention to the lights on the front I noticed this time that the power led was orange but not flashing and numbers 1 and 2 are flashing before it shuts off.

I spent yesterday stripping the machine down and giving it a clean. I've tried running with just one ram dimm and trying different dimms I have. I tried 3 different graphics cards and tested the PSU using the button on the back. PSU seemed fine but just in case I tried another PSU too and no difference at all after any of that. I then pulled the CPU cleaned and applied fresh thermal paste and replaced it, again no difference.

I'm getting absolutely nothing except numbers 1 and 2 flashing for 5 seconds before shut down no matter what I do. I have come to the conclusion that either the motherboard or the CPU has failed. Would this be a safe assumption or is there anything else I could have missed? 

I have no way of testing the CPU as I have no other machines that support my Xeon X5650 so I may have to gamble and buy a replacement CPU or Motherboard. My money is on the motherboard but I'd love some input from you guys before I make a choice. 

Thank you in advance for any help you can give me. :)