Problematic Umhlanga Dealership

I brought in my car to the Umhlanga dealership as the brakes light had come on. I was advised that the dealership was busy and asked to come back a few days later. I did so. It took a number of days for them to replace the brakes which I do not understand. When I collected my vehicle I drove from the dealership towards home in Ballito. Tracker records will clearly reflect this. Before I could even reach home the engine malfunctioned (+- 30km from the dealership). I had to pull over to the side of the road and my car was eventually towed back to the dealership. This occured on the 12th of December. I have had to contact the service department myself on numerous occasions to find out what the problem is. After the first explanation regarding cyclinders and mis-firing, now the story is that apparently there might be a foreign object in the engine. As they had been working on the vehicle they should be able to tell me how the foreign object got into the engine as my vehicle had no such issues before. I also requested them to conduct a full diagnostics check on the vehicle when I brought it in for the brakes and I was informed that everything seemed fine with the vehicle. They have also denied me a loan vehicle/hire car until they can determine the source of the so called foreign object and I have no way of getting back to Johannesburg. The service at this branch is absolutely shocking. A few years ago we eventually took back a car we bought there as the workshop could not determine what was wrong with it. I am aware of management and worker union issues in the Durban area and am wondering if this is affecting service at these dealerships. I am due to get a new vehicle as I am almost out of my motor plan, I am not sure I want to get another Mercedes after this experience.