Hello, I hope the following is of assistance.
There are a couple of concerns that may give the symptoms you describe. First make sure there is no air leak in the intake system, any false air will increase the idle speed of the engine as this is the same as the idle speed control valve opening further, usually if there is an air leak you may hear a hissing noise, make sure both crankcase breather hoses are connected to the inlet manifold, you can follow the hose from the valve cover and it has a "T" in it to split the hose into 2. you can also try spraying WD40 or similar onto the join between the inlet manifold and cylinder head, any change in engine note would indicate an air leak.
There was an issue with the idle speed control valve on this engine, although this was a few years ago it may be worth checking the engine management software, there was an issue where the idle speed was kept elevated a revised calibration sorted this out, you would need this checking by a garage that subscribes to engine management updates though as not all garages can do this.
Finally if possible it may be worth having a diagnostic scan of the engine management carried out, this way any faults detected by the engine management that will been stored can be retrieved and will aid in pinpointing the area of the fault.
As I mentioned earlier start with checking for a leak in the induction system and go from there.
Let me know if you have more questions regarding this and I'll reply as soon as I'm able to.