ford fiesta mark 5 2000 keeps revving when idle?

i have a ford fiesta mark 5 w reg. the car starts fine but when i start driving it wont stop revving. when i press the clutch in as im coming to stop or changing gear it continously revs. i have taken it to a garage to see what the problem was and was told to buy a new idle control valve. after fitting this i was told the problem would be sorted, but the day i fitted the valve and took it out for a test drive the car still continously revved. how can i fix this as it is begining to use fuel alot less efficiently and is rather annoying.