Too Fast but vairable idle problem - airflow?

Hi All,
The car:
1.8i 1991 Mk1 cabby, LHD

The problem:
The car starts fine, idles perfectly from cold but when driving away, and lifting off the throttle to change gears the revs don't drop. Pulling up to traffic lights tends to leave the engine idling at at least 1500 rpm, even when warm. I can pull the idle down by engaging gear and letting the clutch up a little (i.e. like preparing to move off but with no additional throttle). The revs will pull back to normal idle and stay there.

It is as if the throttle is sticking open just a little or an air by pass is functioning.

Any suggestions welcome.

Thanks,
Arn
London UK