My only question is why the centralized blast area. It seems as if there should be more signs of the abrasive. Like cyl scuffing, wear around the top ring, valve seat and stem, etc.
Now, at 1800 hours a cast iron seat will be beat up. For a long hour run such as this... I'd have hardened seats put in. Not just any "hard" seats, but something much harder. Valves are easier to replace than seats to machine. (for one not equipped with the tools) Propane compatable seats are the hardest, cheap, readily available. Other metals will transfer heat quicker, race world stuff, probably harder to find in the UK.
Back on point. Is it possible the blast area is a function of heat? WMO runs hotter than diesel (it should as it contains more btu's) So, I assume? this is the area where the injector spray points. Most concentrated heat area. Also, how is cooling right at the wear point? With the compression valve right at the same area... is that very spot getting hot enough to burn the chrome?
Is it a thermo siphon system, or pumped. Maybe... it is a function of heat.
It seems like the top of the piston should also be blasted if it was abrasive.
Other metals will take the heat more IF that is the issue.
Wags