So could they be wrong?
Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes, Yes.
Garages seem to love saying "its the headgasket", it must give them the horn or something. Did they compression test it, or did they just scratch their arses while they read the Daily Sport? Probably the latter.
If it takes ages to warm up, with no eventual overheating, perhaps the thermostat is stuck open, so the engine's circualting too much coolant round the radiator, giving the exact symptoms you describe.
It could be other [
cheap to fix] things too.