Worst service provider on planet earth

On the 20th of November 2017 I took my car at GP Motorworks (Randburg) seeing that they were BMW and MINI Cooper specialists, RMI approved and affiliated with Hollard who I have an extended warranty with. My Mini, had an engine warning light and would lose power and overheat. I was advised that it was a thermostat issue, and the thermostat had to be replaced, I then obtained authorisation from Hollard to fix the car and I settled the bill at R10,126.00. A week later, the same engine light went on and I returned the car back to them only to be told that it is a different problem that had nothing to do with the work undertaken. I was charged for the second diagnosis, because my problems had nothing to do with what was fixed, although the performance and symptoms were the same. I was told I needed to fix more things and that is when I decided to take the car to BMW for a second opinion. I was informed that there is a problem with a thermostat, of which GP Motorworks claimed to have replaced. I called the workshop (GP Motorworks) and I was told to return the car to them for them to be able to return the thermostat to their supplier as it might be faulty, but if that was truly the case, why did they not pick it up when I returned the car? Why was I charged and advised to fix other things? I advised that I do not trust them again and I’d want to continue with BMW therefore can they provide me with proof of where parts were got so BMW can know to replace them or not. I was hung up on.