Unacceptable service Mercedes Sandton +Mercede SA

I have a C Class 350 CDI that had a terrible noise coming out of the motor and called Mercedes to arrange to bring it in. I was met by a service manager Godfrey who drove the car with me+told me that he would get back to me as there was clearly something wrong .This was on 7th December 2015. He never called me back and after 3 days I called him only to be told that the vehicle had a problem with the gearbox and he would have to get a part from Mercedes SA to replace . I said thats fine Im away from the 11th Jan until 7 Jan 2016 and they have ample time so should be ready when i get back. They never called , low+behold I called them again on 6th of January and spoke to Steven who told me the part had not arrived and he would arrange a hired car but he'll contact me tomorrow. Steven didnt contact me+I called Mercedes SA+spoke to Thakala Rapunga in customer services, he said that my part was arriving on the 19th of January 2016 which he later changed to 19 feb <br> I have to date not received my car + have bought + audi with a trade in on my Mercedes. The trade in price will drop by 7% if the vehicle isnt repaired+ returned by 28 Feb 2016.<br> 3 months to repair is unacceptable, please help?