try "blowing" the engine through with Miller Oils Diesel additive.
Needs a long motorway run (40+miles) 1/4 tank + 1/2 bottle, high revs good speed.
When the crud starts to move it can sound like a shotgun and a load of nails in the exhaust accompanied by lots of smoke...... when I did the vito it almost blacked out the motorway .......... keep going, keep the revs up or you'll stall and it won't be easy to restart.
After another 20miles or more it will ease........... go home.
After you've run the fuel out, try it again..... same mix
When refilling put 10 "shots" to a tank and it will be worse to start with but will improve over a couple of tanks.
The Crap is carbon etc breaking loose and it allows air etc past the valves as it detaches.... slowly they reseat again and performance improves.
Millers at standard rate should then give an extra 10 - 15% mpg adding 3 to 6 ltrs sunflower oil as well can raise the mpg again and keep it smooth (sunflower is a good fuel and upper-cylinder lubricant, with it in low quantities and miller oils additive is in no way detrimental.