I agree Keef,
But I dont know what this self test is that has been performed on the power supply - I have not heard of this before (but thats not saying its wrong).
For £20 for a decent replacement powersupply delivered next day I personally would try that first, however I may be lacking information and if the powersupply does indeed prove true - I would suspect the cables before the motherboard - but the motherboard is the next logical choice.
I only say that because I have never seen such a catastrophic failure of a mother board in my 20 years of commercial progarmming - but that dont mean it does not happen
Would be good to know what you isolate it to in the end.