That's some mileage, Doctor. 'Teddy1975' makes a good point, but I don't think, that unlike Nordic models, a diesel pre-heater was fitted as standard on cars destined for temperate climes - maybe yours does?
A compression test, if low on one, or between two adjacent cylinders, may add weight to your suspicion that the H.G. may be the root of the problem, but I would expect the cooling system to be pressurised as well if that was the case.
I can't really comment on the effect of a fuel additive as I have never used one - sorry.
Regards, John.
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