Hi Shaun

Some meaningful codes and data would be good for sure

2-2.5v seems abit high during cranking for what I am used to seeing on common rail systems?

What is the KOEO voltage?

Have you got a scope?
With no comms and the tests already been done not really giving many clues, it is time to get it out

What type of job does it look like to change the thermostat? Maybe something has been disturbed like the Cam sensor wiring or similar

James