Hemorrhaging money for nothing

I went to Car Service City in Woodmead with an issue that my car is losing power while driving - the diagnosis the first time: to change the clutch kit and to change the brakes, which I paid for. I took it back when the problem persisted and they convinced me to replace the radiator, which I again agreed to and paid for. Afterwards they did further testing and after 4 days of silence, the diagnosed that the car is losing power (miss fire) due to "low fuel pressure which was determined to be caused by the fuel pump and dirt inside fuel tank, which could have come from a bad batch of fuel at the petrol station" and they quoted: Fuel pump R4732 and Labour to clean fuel tank and fit parts R2052. I paid for all the work via EFT and was treated like a ******** because I did not pay extra for the "immediate payment" and the manager told me that he will personally be held resposnsible if I do not pay - this is after I have done business with them over almost a year and already paid over R50 000 towards the car repairs. I eventually arranged for my insurer to tow the car to another garage to get an immediate diagnosis. The conclusion was that the bougie cord set needed to be replaced for a low cost of R800. This seemed to have solved all the problems.