2000 Vauxhall Frontera black smoke !

Help. For the past few months my car has kicked out lots of grey smoke on start up. I've been told it's excess diesel, but 3 weeks ago, I was driving home from work and it started juddering, and then thick black smoke was billowing from the exhaust. The car was still running as well as it does (156k miles), so I turned it off sat for a few moments and started it up again. It started no problem but the smoke started again. I basically crawled home and haven't used it since. I called the aa but that was a total waste of time because he said it was the heater plugs. I thought they were just for starting up. I have also been told it's the injectors. But I have no idea. Can anybody help and is it worth getting sorted. Oh also the engine management light is constantly on, but has been since I bought the car 13 years ago. Vauxhall had it in so many times but couldn't find what was wrong??