Cold start fast idle problem 1.8i '91 cabby

Hi All,
After 5000 miles of driving my Golf Mk1 with 129k miles is starting to give some problems. The facts:
- There is a coolant leak, I think from around the level sensor on the header tank. I keep topping it up at the moment till I get time to fix it.
- Cold & warm starts fine.
- Cold start make the idle go up to say 1250.
- Drive away and drive for 1/2 hour and the idle will be up to 2000+
- A couple of kicks on the throttle drops the idle but it comes up again at the next traffic lights. The car is definitely warm.
- turning off & then again on (the engine) fixes the problem when the car is warm.
It sounds like a sensor to me but I don't know where to look or start. Help much appreciated.

Thanks
Arn

1991 Sportline LHD