Help with my Megane

I have a 1997 renault Megane 1.9 rxe turbo diesel. Recently the head gasket blew. My local garage had the head pressure tested, re-ground and refitted but ever since I have been losing water from the header tank at the rate of 1 full bottle per day (76 miles). I have taken the car back to the garage on several occassions and they have tried checking all the hoses but they can't find any leaks. They have checked that the fan is coming in.
I noticed that steam was coming from the pressure release on the header tank so they changed that but it is still releasing pressure from there. Can anyone let me know if there is a simple fix to this or is there most likely still a problem with the head gasket.