Mark,
The alarm was a 'Maple Commercial Vehicle Alarm', as fitted by BT to their vans.
A witness heard the window smash, the alarm sound and the van speed off "all within seconds". Professional vehicle thieves will overcome most alarms in a surprisingly short time.
I now have an alarm that cost over £250 plus an imobiliser which cuts the deisel and electrics and am concidering a pedal locking mechanism made from a section of steel girder.
Hope I never loose my keys :
John.