Audi is charging more money for a problem they never fixed

Audi The Glen charged me R13 000 to do a diagnosis on my vehicle as well as to fix the over heating. Within 2 days the vehicle started overheating again. I subsequently booked it in again. They are now telling me that it is a different component in the vehicle that is causing the overheating and they need to charge me and additional R5 000. It seems very convenient that after my Car was with them it developed even bigger problems, for instance my heater is no longer working and the car is still overheating.