Hello all, just thought i'd update you on the cause of my lack of power issues with my Td4 03MY in case it helps anyone.
I initially had a split turbo hose whilst on the M6. It was the short 90 deg hose. Symptoms were initially lack of power, and then black smoke and air whistling sounds. Bought a new one of those for under a tenner and fitted it.
Unfortunately it didn't fix the lack of power over 70mph, but at least there was now no smoke. Symptoms now were that on a slight gradient on the motorway the car would start to lose power and thus speed. There was also a slight noise but not the same as the air noise from the split before.
The Td4 turbo is variable geometry and is controlled by the boost solenoid (see black plastic unit in the picture). It applies a vacuum to the turbo via pipe 1 in the photo. When the vacuum is applied the turbo exhaust side vanes close and it uses more exhaust gas to provide more boost.
My solenoid had failed so that it constantly applied vacuum to the turbo, and thus operated it at max boost all of the time. What then happens is that when you get a high speed / load condition i.e. 70mph and a hill, the ecu senses that the turbo is over boosting and derates the engine by cutting the fuel in order to protect it from damage. It will then speed up again when you get back onto the flat.
The solenoid is STC4198, cost £34, took 15mins to fit (don't need undertray off). Fitted and all working perfectly now.
So if you have lack of power on hills at 70mph, try this test. Pull pipe 1 off the turbo and drive a 70mph hill and see if it cures your issue. If it does, voila it's the boost solenoid. With the pipe off it will be a little sluggish at low speed accels as it won't be able to close the vanes for full boost, but it's fine to drive in order to do your 70mph test.
For info pipe 1 is solenoid vacuum to turbo, pipe 2 is vac supply to solenoid from the plastic reservoir at the side of the engine, pipe 3 is the solenoid vent to fresh air via the little filter. Check these for holes as they rub various engine parts. Also worth checking that your boost solenoid filter isn't blocked, as this would prevent the solenoid from releasing the vacuum and cause the same symptom. Fair chance it's ok though and your solenoid is bust.
Good luck !
