2001 W220 (s320 cdI) Black Smoky Exhaust

[ASIM]

Hi, I have a problem with my Merc. I had the car serviced and specifically mentioned to the independent dealer I originally purchased the car from that the car was blowing black smoke. As part of the service the air filters had been changed oil change etc. However the car contains to give off black smoke which can be quite embarrassing and no doubt the drivers behind me think they’ve entered a nightclub….. Had the car MOTd from another dealer and they said the smoke level was normal and advised I should try some injector cleaner. I’ve tried this but to no avail. Still blows black smoke which is worsened if I try and pull away from traffic lights/roundabouts quite quickly. The exhaust tips are charred as a result and the external boot area needs constant cleaning. Please help!