Fiesta 1.25 (N reg new shape) revs when clutch depressed

[red-ranger]

Please can anyone help? - my Fiesta 1.25 Zetec (N reg new shape still running beautifully until now...) has developed a fault even the garage doesn't know how to fix. When the clutch is depressed, either to change gear or when going at a steady speed the engine races like mad until the clutch is lifted. Seems worse the higher the revs, i.e. changing 1st-2nd or going 70ish. Makes changing gear rather tricky! It doesn't happen at all if the clutch is pressed while stationary though sometimes the idle is a little high (1500) though this usually rights itself. Engine feels a bit racy at high speeds though doesn't seem to be revving that much higher than usual (4000ish at 70+). Idle has been a bit unsteady when sitting still for a while though not so bad as to worry about.
The garage changed the idle valve last week but that's made no difference at all.
Can anyone help or know of anything I could suggest to the garage? Thanks all ;-)