Astra 1.3 CDTi flat running

[Skeksisss]

Car is a 2007 Astra that runs fine above 2500 rpm but flat below that.
Live data shows little change in the turbo wastegate position irrespective of what revs / throttle load applied.
Wategate arm moves freely by hand but on start up opens to about 75% and then doesn't move when running.
No fault codes are present at all - ready using Snap-On professional reader.

EGR valve seems to be operating fine.
Boost valve (in wheelarch) has been replaced.
Wastegate arm moves freely.
Induction system crud has been removed.

Anyone got a logical solution to this - I don't fancy giving to a main dealer and have them swap out everything a huge cost until they get lucky.

I'm guessing that one of the sensors is giving an "in range" but spurious figure and making the ECU think boost is too high.

[scarble]

Can you read manifold pressure?
of course it's a 1.3 diesel.. are you sure it hasn't always been like that?
another thing to check, does it have a turbo coolant temperature sensor? bit of a shot in the dark..