Really! The only root cause of black diesel smoke is a lack of burnable air.

GNVQ level simplification. Most diesels will smoke long before the engine reaches stoichiometric ratio (ie lack of 'burnable air' whatever that is). The root cause is low fuel injection pressure leading to large droplet sizes and poor utilisation.

Unless sub-standard lubricant is used, all other PD cams are bullet-proof.

As far as I'm aware, all those engines used the same profile camshaft. The narrow lobes operating the valves are an inherent design problem. That is why VW are so particular about using oil of ther correct standard. valvetrain lubricatin on those engines is marginal.

or indeed any other VNT-equipped vehicles

Except my experience of indusrial VNT equiped engines that push 20+psi all day long?

Clogged vanes are a symptom of excessive soot/oil in the exhaust and not a sign of metallurgical failings of the turbo. The vanes can be cleaned and restored to correct operation.

Your constant misdiagnoses must have cost any that made the mistake of following them many hundreds of pounds.

As opposed to all the money you save by replacing a perfectly functional turbo? Garrett and Borg Warner make superb turbos that survive under far heavier duty than you're ever likely to find in a passenger car. Any statement to the contrary is baseless libel. Turbos fail because of poor lubrication, cooling or over speeding. That is a fault of the operator of the vehicle or the manufacturer of the engine.

as you've clearly never handled one; cam belts are not made of knicker elastic.

Might want to look at the PD cambelt a little more carefully. The teeth are not uniform al the way round the belt-because of the huge stress imposed upon it by the extra injector cam lobes.

thanks for the advice , sorry u got into an argument !! , still got the problem ...... in the mornin it happens when i turn onto 1 road half a mile from my house then when i leave work it happens straight away !!!!! AAAAAAGGHHHH . why why why !!!!!!!