Chris might well be on the money here as its not unusual for head studs to snap.
However, I have had one work loose on my own SS and I managed to tighten it up by fabricating a tool to do the job, its still a pain to get the right torque.
I'd of thought the Mk4 would have the uprated studs fitted, I think there's a difference in colour of the stud from my (feeble) memory.
Oddly enough, there's a proper tool for sale to do the job here:
http://www.ebay.co.uk/itm/260976661597 Good luck - Frank