Cylinder Head repairs at 140000km - REALLY!!!

I have a Blue Efficiency Compressor C180. During the motor plan/ maintenance plan I complaint that the car was jerking and when I start up it feels like it is going to cut off. The coolant error message light came on quite frequent. It took them 1 week to get my service records after I complained to MBSA. They don't have any record of my complaints. After the motor plan was over and the car could barely move and now the engine light was on. I was told that I need to pay R18000 to repair the cylinder head. Repairs to the cylinder head at 140000km is a mechanical fault and not wear and tear as the service manager claims. They had replaced the oil coolant and the car was fine for a week. Taking the car to them they diagnosed that the left booster cable and the radiator cap needs to be changed. I did not do the repairs because this was the 3rd time the radiator cap needed to be replaced. After two weeks took the car back to them, this time the diagnosis was different. As a consumer do I need to pay for their misdiagnosis? I had to tell them what was wrong with my car. I'm disappointed!!!! I thought this is a BRAND!! cant go wrong. Are they buying inferior products.