2006 Skoda Octavia head gasket?

Engine Cooling problem
2006 Skoda Octavia Front Wheel Drive Manual 120km miles

i recently had the timing belt and water pump changed on my car. Since this the coolant levels have been dropping rapidly. I have had my car in a number of garages ( including a recognised skoda dealer) and following extensive work including pressure testing no leaks or problems could be found. Another garage took in my car and said it was the head gaket. He skimmed the head and said the problem was sorted, but its not. The coolant is sill leaking. Have you any idea what could be wrong? Its a 1.9tdi.