Big Matt, How long has the car been like this? Did it suddenly go wrong or has it deteriorated over a period of time. Have you owned the car from brand new, do you know the history of the car? How many miles has it done? Regarding the VNT mechanism in the turbo, the vanes move to increase the boost at low revs and move back to decrease the boost at high revs. The idea is that you get plenty of pull from the engine across the whole rev range. Older diesel engines with fixed turbos suffered from "all or nothing" where the was no pull at low revs and then they would suddenly take off as you accelerate above 2000 rpm. If the VNT mechanism sticks it usually results in too much boost and the engine trips into limp mode and you get a fault code.