Three out of four holes were usable with the original M6 thread.
An M8 bolt was centre drilled (as suggested on here) and wound into M8 Helicoils near the top of the bolt holes (this area was originally without threads). So the long series 1/8” bit was quite well centred, then went up a mil or so. At this point it was determined that the holes weren’t actually filled with bolt stumps but a mixture of swarf and diesel “coal”.
Picks, solvent, compressed air and a chasing tool made from high tensile M6 rod were used to remediate the original threads.
Except 4, which was stripped. So the Helicoil will probably suffice for that unit alone, with the first three using factory style fixings.
I’m going to order a specialised repair kit as a backup but reckon we’re onto a fix as it stands. Injector seats refaced tomorrow and hopefully a running Vito by Monday night.