Hi Andrew, I am still not sure it can be "major" as the engine turns over freely. Most things like a broken con rod, knackered crank, etc would cause a lock up, or at the very least, make horrible noises when the engine is turned over..
Piston rings would not have stopped the engine, I have had loads of them go in bygone years and had to rebore some engines, but they still ran...
There are not many things that can stop a diesel engine from running really (excluding ECU stuff) the main one being lack of fuel. That is where I would start my search. Check the fuel supply chain working backward from the injectors. Then if that all proves ok, check the timing of the injection and the pressure in the common rail.
Good luck with the car, I hope you get it sorted quickly and cheaply.
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