Poor Workmanship (Negligence) on my Vehicle

My CAR: Chevrolet Cruise 1.6 LS 2010 (159 KM all service conducted by GM from 0KM) was taken to the dealer because there was a big sound in the Engine that caused the car to lose power and not move. They have book it in and after their diagnosis they discovered that the Valves, Rollers and Cylinder need to be replaced, they quoted me R21195.87 that includes labour, and all the parts that needed to be replaced and promised that the car will be ready and perfect after the service has been conducted. During the Lab period the Tapped Cover of the engine was damaged due to negligence and they kept it away from me not informing me about it. After a month they called me to pick up the car and I have paid the requested amount. On my way home after 10KM a realize that there is a huge oil leakage from the engine, furiously I took back the car in the morning and they mentioned that the Tapped Cover need to be replaced and again they want me to pay the value of R3000 of which the negligence was not from my side. The Car is currently at the dealer now and they do not want to be held responsible for that.