Black smoke is overfuelling so is unlikely to be an injector. If you have some sort of filter in your turbo pipes take it out. It's restricting the airflow. Take it on a drive. If it's back to normal great. If not......
It sounds like either split turbo hose, blocked air filter or fecked egr valve.
Get somebody to rev the engine for you. Can you hear the turbo whistleing? Check the air filter. If it's clean then put it back in. If it's very dirty replace it with a new one. Feel and listen all along the turbo pipes. Can you hear a hissing sound or feel an air leak like you have a puncture? If no can you feel the rubber pipes expanding and contracting as the turbo cuts in and out? Egr valve. Get shot of it. get one of these egr removal kits.
Land Rover Discovery 300TDi HD EGR Removal Kit + Hose on eBay, also, Land Rover, Car Parts, Cars, Parts Vehicles (end time 17-Dec-08 19:59:18 GMT)
Are you around during any daytimes this week? I am on nights this week so if you are around during the day I'll see if I can pop down if your struggling to sort it.