Hi

It could be a number of things, I would check the EGR valve first as that is quite common on the HDI EGR "exhaust gas recycler" reuses any unburnt fuel in the exahust gas and forces it through the engine to keep your emmissions down and improve economy. often these get mucky so you can remove them and spray them with carb cleaner! do not use anything else to clean them.

To check also any air pipes that have splits. tears, rips etc have a feel around the underside of each air pipe as again this can ause that fault.

Lastly check the air mass sensor going into the air box, I cannot reacll where it is on that model but they are very obvious once your looking at it. normally a wire going into the main air intake of the car, unplug or unscrew it and again clean with carb cleaner and refit.

try these before you pay anyone, lastly it could be an injector leak sucking in air, you can test this if you can get to the injectors, poor a little water around each one and see if they blow out any water or bubble up and **** the water in, this can be a cheap fix but the labour invloved may be expensiove, in all honesty buy a haynes book as its pretty simple just takes a while.

Hope this helps, if you are unsure what I mean then get back to me and I will explain.

Regards
Dan