Forsdicks Sandton are useless when it comes to service or technical work. I have bought three cars from them and my sister has bought from them. I would gladly buy from them again. However, I would not take my car to them for anything more than a basic oil change service and, on occassions that I have, I've been thoroughly annoyed.
I generally use Lyndhurst Auto (at Melrose Arch) nowadays. Sometimes Hyde Park Auto in Sandton (next to Aston Martin) for minor stuff as they're much closer, particularly if you normally deal with Forsdicks.