Thanks Steve, you pose some searching questions which prompted me to clarify the answers with the customer a few minutes ago. The heavy smoking appears to only happen whilst the vehicle is stationary at about 2500 RPM (as you suggested). This is accompanied by a misfire. He generally only travels during the hours of darkness and he has not seen any evidence of smoke whilst driving. The smoke smells heavily of diesel and is blue/black in colour. His MPG is poor approx. 28 MPG (after filling up today.) The smoking and poor MPG is his main problem and it is doubtful if it would pass the MOT smoke test. This is his second TD4 and never had these problems with his first (02) vehicle. The rest of your questions are covered in the update.
UPDATE: We did not have a donor car to play with today but we did the following. The turbo actuator rod moves fully when you blip the throttle. The turbo hoses expand also (before and after the intercooler). Applying vacuum to the MAP sensor shows exactly the same reading on Autocom. Now the interesting bit. The inlet manifold was removed and was badly coked with baked oil. It has a strange set up with 4 inlet manifold ports and 4 ports to the rocker cover. 6 of these 8 ports were covered in oil. The depression valve was removed which has a spring loaded diaphragm and not knowing how this should be tested, it has been now been replaced. The inlet manifold has now been thoroughly cleaned and the whole lot will be re-assembled tomorrow upon receipt of new gaskets and seals etc.
I await with bated breath.
Regards
Pete M