Poor service at Bosch. Its almost a ****

I took a my car to Bosch at BOSCH Service, Selby Ext., 14 Hans Pirow St, Johahannesburg on 14th of September because it had issues with the funny noise that I suspected came from the tension-er/ tension belt. The car was also losing power(I suspect that it had to do with the faulty sensor as it kept on displaying a message that I must check the coolant while it was still fine ) and it was displaying a lot of errors on the screen . The owner promised to get the car fixed within a week and at a total cost of R7000. I agreed and paid the full amount and told him that all those issues need to be sorted out. 2 weeks later the guy told me that the noise I needed to be fixed comes from the engine and he will have to strip the whole engine and it will cost me a huge amount. I was very ****ed off because I brought in the particularly to get the noisy sound fixed and to sort out the power loss issue. He promised to sort out the noise, power issue and clean up the errors for R7000. However to this day the car hasn't been fixed, only the starter(he put in a new starter and I was willing to buy a second hand starter) was replaced. I accepted that hes not going to fix the noise issue and asked him to fix the rest (loss of power issue) as agreed but he hasn't done so. Hes always asking me to give him 2 days to sort out the power issue as it has to do with sensor and hes visiting other garages to get it fixed. He doesnt even pick up my calls or return my missed calls. They are extremely dishonest and ***********. I am even questioning the s****s of their mechanic technicians because they are worse than the street technicians. Please advise what I need to do about this issue.