sovereign BMW vereenigng *********** service department

i took my car in for a engine overheat and drivetrain malfunction warning that come in when i drive on the freeway and explained this to them, they insisted i pay for diagnostics and came back with a lot of things. i asked them from the long list which ones are contributing to the engine overheat and drivetrain malfunction, they mentioned them and i authorized the repairs they did extra work without seeking authorization and i refused to pay for that. I took my car and first time i hit 120 km/hr both warnings came back . i then took the car back to them they demanded i pay another diagnostics fee claiming that it might be the warning but the problem originating from a different place, where might it originating from this time the service department had no answers. The technician took my car for a test without my consent and a tire burst they said i am liable because the tire was worn, so my question was if it was worn why drive in the first place why was the inspection not done before testing it. I lost close to R10 000 because of their incompetence i would not advise anyone to trust them with their pride and joy.