This is the problem with modern diesel engines, irrespective of the manufacture.
They all will suffer this problem.
As you know you can get into a higher gear early in the RPM range.
This gives us the opinion that you will save fuel by increasing your mpg.
This assumption is wrong, use your gears increase your rpm, run hot,hot this then helps clear the the build up of particles.
The other thing to do is change your oil more frequently, go to euro parts, buy oil filters and 20 litre drums of oil, change every 6000 miles cost are pence per mile.
Short cool slow low rev travel is the modern diesel death call.
A tip we were given was to run at 3000 rpm for 30 minutes.
Did clear the fault , but you need to change your driving habit to keep it clear.
Maybe i am typing *hit that others will disagree with, but it did work for me.