finally found actual evidence tonite :
the circle shows the exact spot where the heads go porous. Although the camera can't pic it up, there is actually a crack which runs from the corner of the large water gallery, up to the oil gallery above it, at a slight angle.
Well done Mongo for spotting it - it was very hard to see because the bandsaw that did the chopping has dragged the alloy across the crack and disguised it very well - but when you see it, it's obvious - it's a bit like one of those picture you stare and stare at, and suddenly see a Dolphin riding a 747 or something.
It's a classic stress crack - brought about by the compression of the headbolt - if you imagine the clamping force nearby running vertically downwards - that place is exactly where you'd expect a fracture.
The crack is also present on the other half of the head too.