Okay, so I have narrowed it down to either the CPU or Motherboard. Problem is, I dont have a spare CPU to test in my rig...
I removed/unplugged all other components until it was just the bare PSU, Motherboard and CPU plugged in, but it was still stuck in the 1 second boot loop. Tried a different PSU, same thing happened. So it can only be the CPU or Motherboard...
Not sure where to go from here now?