It is a fairly common problem with the cosworths. What might also help if cleaning out the ISCV as they can get crap in them. It might actually be the ISCV causing your problems. Turn the car on and unplug it and you will probably notice the revs drop to what they should be. However it does serve a purpose and not running it might cause other idle problems.

I have had a couple of problems with idling but they seemed to be short lived. It is certainly something to do with the throttle body and or it's surrounding parts.