Check ingition warning when revs drop to idle and turbo stops working.

[tobyd10]

Hi,
I have a 2007 1.9 DCI Scenic.

I have read lots of posts about the P0235 code (that I have) and the various causes and remedies.
Haven't found a post with exactly the same symptoms.

If I start the car and don't let it idle and move off keeping the revs up all is well. Once I let the revs drop to idle I get the Service/Check Ignition warning and the turbo stops working.
I have no smoke etc. and have just done a 1000 mile round trip to France 2 weeks ago. I gather this could be anything from the boost sensor to a split hose? If so is there a more likely place to start?
I have today taken off and cleaned the EGR valve but no change.

Would a garage be able to run a better more specific diagnostic that would tell me what the problem is opposed to the generic code reader?

Many thanks for any help much appreciated.
Toby