Poor Service & They Damaged My Car

On the 02/07/2014 I took my car Mazda 3 to Car Service City Omonde as my car had a problem with clutch kit. I suggesred they do major service as well total amount i paid was R13 219.26. On the 25/02/2015 my car broke down due to engine failure,I took it back and their mechanic told us the engine failure was caused by greasy dirty old oil & he confirmed that the oil failure caused the damage and agreed that the mechanic that serviced the car did not drain and change oil when they did the service.The manager Mduduzi said they wont take responsibility for the current problem as 6months guarantee was over & said they can diagnose the car but i should pay R2000.00 for diagnose and once diagnose is done they will give me a quatation that i need to pay.I took my car back as I feel it is unfair for me to pay for diagnose and quatation as the damage on my engine was caused by their negligiance as they did not drain and changed oil when they serviced my car. I am so disapointed from the bad service i received from them.