Thanks for the reply Brian.
The car is a sportline model and has had no suspension work done recently but you have reminded me that I reversed over a soft plastic bucket on the drive the day this started. I have looked at the rear suspension and can see two switch type devices affixed to the lower suspension legs? they face towards the rear of the car. The switch arms which protrude from them are as follows; passenger side seems firm and positve but the one on the drivers side is loose and seems basically to be broken, the arm is loose on the switch body and just waggles about. Now I believe I have broken this one with the bucket.
Do you or anyone else know the correct name for this item or Mercedes part number? Thanks again, Karl. ( the car is left hand drive and I am in the southwest of France ).