Excel revs too high

I recently bought a manual 2000 Excel Sprint. It runs ok, except when
the engine is warm the revs stay high after I put my foot on the clutch
- eg. when approaching traffic lights.
It takes several seconds before the revs start to drop, and after 10
seconds, maybe more, the revs eventually get down to a reasonable
idling speed.
I checked the mechanical linkages under the bonnet, and made sure the
throttle returns easily when the pedal is released, but this doesn't
affect the revs.
Is this something in the EFI? Can it be fixed by me, or is this an
expensive woprkshop job?

Thanks for any input,

Barry