2006 Mercedes Benz C Class engine problem

whilst accelerating into a lane of moving traffic, I heard a low sounding bang and a large plume of smoke came from the exhaust. I continued driving albeit slowly. the car appeared to perform ok but there was a noise like a leaking exhaust except it was rotational in time with the engine.There were no lights showing on the dashboard. I took it to small one-manband merc specialist who after using a stethoscope thought it might be a faulty fuel injector on no 4 cylinder. He replaced this but whilst driving home the problem got a lot worse with loud knocking noise from engine and the engine kept stalling during acceleration until eventually it slowed to a halt. The car was eventually towed home on the back of a greenflag waggon.
The car is normally driven by my wife and covers approx 6k miles per year