belt tensioner

Weird one this. (diesel by the way.)

Pulled onto forecourt and checked tyre pressures reversed round to get juice and stalled going forward. restarted and went forward , no power steering and alternator fail warning!
Crawled to pump filled up and drove away OK.

Definite slapping belt noise so off at next junction and round to my friendly mechanic.

Take a look and the belt is slack

Out he comes and '****s' the offending belt tensioner and lo and behold the belt is re tensioned.

Anyone had one of these stick ? should I replace it?

It is partially seized on it's pivot. Needs taking off cleaning and greasing.

Lost mine, but that was down to the crankshaft pulley coming loose! odd, not done it since.