Review service at BMW in Pieitermaritzburg

Took our 320d (2010 model - 206 000km)for a service as the service light on the dash was appearing, but car actually fine and driving well.
Long story short, we still dont have the car back after 4 weeks; the car having gone back twice during that time, them telling us that they put it on their machine and found nothing wrong the first two times. Now we're told that the engine is blown and it will cost R100 400.00 to repair. the car wouldn't have even sold for that price.
Shocking service , buggered our car up, (denying it of course) and now we have to pay?? bull....