IN JUST four short years, the C-class has firmly placed Mercedes at the top of the premium medium car segment, comfortably outselling the BMW 3 Series and Audi A4. To help keep its best-seller on top, the Germans have introduced a facelifted, improved version with fresh diesels, seven-speed autos across the range and a host of safety, driveability and functionality enhancements that should keep the opposition scrambling.