Air con pulley tensioner

[GaryNoGrip]

After changing spark plugs/ht leads and running shells finest again, am able to push the car up to 6k with no problem, hopefully this has cured my problem( will find out Saturday as re booked on to srr( thanks again Charlie)) after the run I parked up and let it run for a bit, couple of minutes later heard a horrible noise from under the bonnet, switched off engine straight away and found that the air con belt had come off, luckily j666eds was with me to help get it back on again.
The tensioner pulley seemed fairly easy to move though, can this be tightened or is it foobared?

Thanks Gary

[GaryNoGrip]

103000 miles says it doesn't mind lol!

[GaryNoGrip]

Thanks all, should this be a priority now it's come off once, will it be more regular?

[GaryNoGrip]

Ordered new pulley and belt, thanks for the help!

Gary