As said above IMRC sounds like a likely cause, at around £180 for the part and an arse to fit its not a cheap fix if the gears are all mashed up, sometimes part of the electrics to this unit fail and this can be fixed with a 99p parts and the cost of your own time. Check the manifold runners are being switched when the engine is started by looking at the lever on the end of the inlet, this should move, sometimes the clip snaps off the end and can be fixed cheaply but 50% of the time the IMRC unit itself goes faulty