Thanks Scott,
Well Trevor - great minds think alike. Although in my case it could be idiots always agree!
Seriously, this looks more like a B&Q ordinary mild steel rough-as-a-badger's-backside threaded screw rather than a high tensile set bolt. Probably simply sheared at some stress raiser point along the thread where these threads are rough.
Surely, surely, this is not a VAG sourced part ( please tell me it's not Scott), but looks suspiciously like it may have been made very far east of the English channel and found its way to the local motor factors or an EBay listing.
Really interested in this, since I'm sure someone else has had a similar experience.
Kind regards,
Gareth.