Spent some 2 hours on this problem today. Printed off diagrams from Microcat for the turbo vacuum pipes, EGR vacuum pipes, engine mounting vacuum supply and the main vacuum pipe from the vacuum pump to the brake servo.
Removed the inlet manifold and had a close inspection of all the pipes. All connected correctly. Checked the vacuum pipes from the main vacuum supply to the turbo reservoir and turbo vacuum valve on the right side inner wing.
The pipe from the reservoir to the turbo vacuum valve and the pipe that goes to the turbo from the turbo vacuum valve were reversed. This means that the turbo doesn't get the right amount of vacuum when it needs it, it has full vacuum all the time, causing the VNT to close up fully thus choking the engine when it needs power (hill climbs, motorway speeds, etc).
Tried it up a steep 1 mile hill that we have nearby, it was so fast I had to brake sharply to go round the corners at the top. On the motorway, what a beast.
Lets hope that has solved it.

Steve. 1968 Series IIA Pastel Green, nearly there project

Result and didn't cost too much...  Pete

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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

I had a similar problem. The TD6 started to feel a bit sluggish in January. The revs wouldn't go past 2200 without pushing the gas pedal to the floor. The revs would then jump up to 3000 and beyond and the car would move then effortlessly. No error codes at the time. When I had the car serviced in February by the same indie garage I have been going to for the past 6 years, the mech said that this was due to the nature of the EGR and completely "normal". The car also passed the MOT without any issues.

In April, the car started throwing an "unknown code" P1E30 on the iCarsoft diagnostics. The performance of the car had been equally bad since January. Google indicated that it was turbo related (Boost Control). I had previously checked all the visible tubes and hoses for any cracks but they were all intact. EXCEPT one fabric covered tube (10 cm in length) which I simply plucked out from the boost control valve. The ends of the rubber tube had deteriorated to crumbs. Of course this wasn't visible due to the fabric. I replaced the tube and performance was back to normal instantly. The error code doesn't show up anymore either.