Ongoing pathetic service from mechanics

For the longest time my Mercedes ML has gone to this dealership for a problem with the gearbox. Not once did they repair the gearbox but blamed everything else for this fault. Last week Tuesday the car went in for the same problem we were experiencing for years. Even though the car was left there the evening before they had done absolutely nothing just before lunch time when we called to hear what was going on. Not long after that we received a call from them stating (again) that there was \nothing wrong with the car"and it could be fetched. We told the service adviser that it was impossible and he agreed to have the car tested first thing on Wed. morning. On Wed. morning they decided the talk converter was faulty and had to be replaced. As usual they did not have any parts and said it would be fitted on Mon. On Fri I spent 2 hours on the phone to stop this train smash but Not one of the 4 relevant people were in to take calls for a 2 hour period. This morning @7am I was there to stop this disaster. I was told they had started fitting a new talk converter. I told them to stop immediately. They simply ignored this only to break another part that was perfect before!"""