help please.300 tdi misfiring

hi all.my disco 300tdi just started running rough very suddenly.i got it home and checked it all over for anything obvious.its not preasurising the water or losing any water or oil and its not breathing heavy out of the rocker box.i cracked the injectors 1 at a time and they all slowed the engine except the last 1 which made no difference at all.could the injector have gone so suddenly?
its also spewing out black smoke when revved

injector ****ed

Try soaking the injector in a cleaner such as petrol. It might clear it. However, replacement injectors for the 300Tdi is relatively cheap. New they are £65 and second hand, can be bought for £25.