The reason the belt wont stop squeaking is that its broken, or it is now!
I slacked the bolt and started tightening the tensioner but the arrow marker was'nt moving towards the measuring point
then - Crack! It was turned a turn too far....I just got home (in my van) and guess what? the parts dept of my local Merc stealership closed 20 mins ago...Bah.
Whatsmore the car is 10 miles from said dealer...how far can you drive a beltless Merc 190e before something nasty happends?