Hang on a second chaps !
If the garage has offered to replace the HG, thinking, rightly or wrongly that the HG is at fault, then they are taking responsibility. IF therefore they replace the HG at their expence and its found to be something else then surely they will have to replace that also.
The buyer has found a fault, the garage is admitting liability, let them sort it. Granted also you'll be without it for a day or two, but to me seems like a good solution.
Granted also that the 200 mile trip is a pain in the ass, but least the poster will have some fairly big jobs out of the way at no cost to him.
I think the problem here is "how does he do 200 miles" with a suspect head gasket, and NOT give the garage an excuse to "blame" him
I would want written instructions from the garage, and full acceptance of liability before undertaking such a journey.
Yep, and, in addition, it's more than likely not the head gasket so you'll end up with a head gasket fitted by a garage of unknown provenance rather than the one fitted on the production line where they get it right every time.
However, if it were mine I would not close the door on the option of the garage sorting it or taking it back, hence don't start pulling it apart.
Phone the garage, tell them you'd like to top it up with coolant and see if it's a one-off airlock that's now sorted rather than make a 400 mile trip needlessly. If they agree, run it up to temperature, check for obvious leaks, take it for a drive with some water in the boot just in case, check for leaks, etc....
OR, if you're not confident in your abilities, phone the garage and ask if they'll pay for a local (to you) garage to double-check it before you make the journey rather than risk a wasted journey or further damage.
Kevin