Son's pals at Maplin kindly explained that the heatsink being wrong way around had left enought of a gap that the CPU fried, so I had to buy another at full price.
I had the same problem on an AMD system I built a few years ago. The clasps on the heatsink / fan could only be installed one way around due to the vicinity of the PSU.
The processor didn't fry but ran at between 55 and 75C, shutting down due to overheating on intensive tasks. It took quite a bit of metalwork to seat it the right way around (modifying the clasp).
The case had clearly only been tested with Intel architecture Mobos...
