Sako83, how did you conclude that the mechanic who replaced your head gasket was an "expert Mercedes mechanic"? Does he guarantee his work? If so, I'd ask the second mechanic (who verified that you have a problem with the new head gasket) to call the expert Mercedes mechanic and explain his diagnosis. Then have the expert Mercedes mechanic perform whatever rework is required to make the job right ... at his expense. For future work go elsewhere, maybe to the second mechanic.

__________________
Fred Hoelzle