Sounds to me like that was good advice.
If the car gets "up to speed" in any normal way, then sure as God made little green Landies, the head gasket is probably OK.
Chief suspect is somewhere in the FUEL SYSTEM.
Suspects include
one or more "leaky" injectors - they can dribble fuel into the cylinder(s) long after you switch off. Eaasy to check this after taking out injectors. One minute each on a Hartridge tester ....
injection pump needing attention, or maybe some previous "expert" has "adjusted" it?
Things like that, but for my money I would go for checking the injectors first, fitting 4 new glowplugs at the same time (get good ones ...) and as that only cost about £30 so far we might be in business.
If you were near here I would do your injectors for you ...
CharlesY