If the car passed the emission test, there's not much to worry about. Carry on regularly servicing the car.
I've had high mileage diesels before and just before the MoT I would change the air filter, put in fuel treatment and give the car a hard drive. The emissions were always on the high side, but low enough to pass.
Few weeks before the MOT I did use the diesel cleaner twice and That has stopped the black smoke. I am a bit worried if it is the fuel injectors/fuel pump which is very expensive stuff.
