After following this tread & having a few of these to do I chased down the Contitech & Gates technical service to find out if they knew of this problem or the VAG TSB. Contitech & Gates both have technical bulletin on this problem. The Contitech TSB extract is listed below even though they don't supply the stud in their kit

Failure of the belt drive as a result of the tension pulley breaking off
after breakage of stud bolt CT 1028 K2/K3/K4, CT 1051 K1/K2
(VW/Audi TDI PD 1.9/2.0 L)
Power Transmission Group
Automotive Aftermarket
ContiTech Antriebssysteme

Concern:
Belt drive failure soon after installation
of the kit due to the tension
pulley breaking off.

Cause:
The tension pulley was unable to gain
full supporting purchase on the
engine. This resulted in the action of
lateral forces on the stud bolt, which
subsequently broke.

Remedy:
The stud bolt must always be inserted
as far as it will go into the engine so
that the screwed-on contact surface
of the tension pulley can be fully
supported on the cylinder block
without any additional loads being
generated.

http://www.contitech.de/pages/produkte/antriebsriemen/kfz-ersatz/werkstatt/service_einbautipps_12_029_en.html