Thanks for that. It is plumes of black/blue smoke at about 2 - 2500 RPM accompanied by a mis-fire. A destroyer in WW2 laying a smoke screen would be proud of it! The garage who are better versed than me on diesels tell me its diesel smoke which can be smelt.
We have doubts about it being an exhaust blockage because the performance is good and there appears to be a good flow from the exhaust system. Only the back box is left intact (cat and middle box removed) and I cannot see what can block the back box. However, it has not been discounted but we will focus our attention on the boost pressure regulator first.
Regards
Pete M