I'm in the process of doing the cylinder head on the 300tdi.
One thing I have learned is when doing the initial 40nm torque down go over the bolts at least 3 times, still at 40nm. I found it took 3 times before 1-6 bolts came up to 40nm. I think that caused the gasket failed in the centre in the first place. The centre bolts took at least another 60Degrees before they came up to 40nm.

Cheers
Dennis