It could be one of two things. Under your throttle body you'll see a cable attached to a pivot, make sure the tab the cable hooks to isn't broken off which is a common problem. If it's there, then the imrc motor is crapping out. You can search the forums and find info on how to pull it apart and clean it, or just get a new one and be done with it. If the tab is broken, you can get a new plastic one or companies like CFM sell billet upgrades, although getting that shipped to the UK may be on the pricey side.
2002 Focus SVT CD Silver/Blue Inserts
#695 of #4788