Well, I got my motherboard testing card (thanks for the tip, bit-twiddler)and tried it out on my dead PC yesterday.

It is currently suggesting that the CPU is not running (PSU checks are all OK), although the CPU does start to warm up to the touch. A friend of mine has some spare, compatible old CPUs and a couple of machines I can fit my suspect CPU into so some more component swapping/testing will be done hopefullly on Monday.

Could well be a dead cpu or it could be that the clock is not running. The former is simple to repair, just change it. The clock is not so simple.