Okay, here's what I tried - I sprayed the v belt with some water while the clutch was engaged and it actually made the belt slip with a loud piercing squeal and the compressor pulley coming to a halt. Thanks nachi11744 for that simple tip.

So it's obvious that the belt needs to be tensioned a bit but if this fails I will replace the v belt (I can't find any record of when the v belts were last replaced...) - looking at the set up it looks straight forward but I'll just have to read up on how to do this first...

Cheers,

Ysmael