Only ever use Superdiesel and stick to the same brand. After starting the car stay in lower gears keeping the revs to between 1500 and 2000 for the first five to ten miles (don't bimble along at 1000rpm). Take your car for proper runs of at least 50 miles outside the city every week. Take it for a 40 mile run using plenty of revs immediately prior to the MoT.