Cannot slowdown tickover

[Martin Prior]

If your car has a Zenith carb it may be drawing air at the carb to inlet manifold flange. It's a regular problem due to the soft alloy the carb is made from.
Take the carb off and rub the flange on some emery paper on a hard flat surface.

Either that, or the throttle spindle is worn. (also a regular)

I'd put a worn throttle spindle at the top of my list.  Try shutting the throttle further by directly turning the arm on the end of the spindle and at the same time check for unwanted movement.