hi paul
we had this happen to us about 2weeks ago we had put new belt on about a month ago came back with belt slack after talking to a volkswagen tech he said it was a known problem when the belt had been changed more than once vw changed the diameter of the stud on the 2litre and when you tighten it 20nm plus 45 degrees it stretches the thread in the head they always remove stud and inspect if in dought repair we have learnt the hard way it cost us a head hope this may help someone
mark