OK Kenney,
Thanks for your help. By the way, my car is manual gear box. What's happening at the moment is this:
When I start the engine up, it runs at about 1600rpm (As I said, it doesnt go to 2000rpm and then drop to 1600rpm any more, as it should). Then the engine speed gradually comes down to 800rpm, so the waxstat is OK.
When I am driving along and the engine temperature is hot (normal), its OK for a while then you can feel the accelerator pedal push back up, which is the '3/4 vacuum box' going into the decleration position (push rod totally withdrawn). I can see whats happening now, this is allowing the throttle to go back too far and stall the engine. I have tried your recommendation of adjusting the nut (13mm) on the back of the 3/4 unit and have tried several turns in both directions and it makes no difference to the idle speed at all. I can just about get the engine ticking over at 850rpm before the throttle closes completely, to see that the push rod is nowhere near the throttle lever.
Can you help me any further please?