Inabiliy to do repairs

I recently took my Jeep Cherokee to Adco Diesel city as it was smoking black smoke and had a knocking noise in the engine. (I drove the vehicle into the workshop.) they said that my injectors were the problem and needed to be reconditioned and they were the cause of the smoking and as they called it diesel knock. And I accepted the quote of R10000 to do the repair. After numerous calls to them to try and get my car back from repair I was told the car was not starting. Roughly 2 weeks later I got the call that the car was ready ( I was told it was a 1 day job in the beginning). I collected my car on the Friday afternoon and before I had driven even 500mt it was obvious that there was no difference to my car. On the following Monday I took the car back and took the foreman Pieter for a short test drive he agreed that the problem had not been resolved. He said he had another clients car coming in and he wanted to try the sensors from that car on mine and he would phone me I then took my car back 1 week later and they still couldn't fix and it still not fixed and tried to charge me for a diagnostic test again. THIS SHOULD HAVE BEEN A COMEBACK REPAIR AT THERE COST