Is the problem that the idle speed is too high or is it that the idle mixture is wrong?

If the idle speed isn't the same whether the engine is warm or cold, the culprit is most likely the auxillary air valve (the jigger with two rubber air hoses and an electrical plug attached to the left hand side of the cam cover). They do have a small level of adjustment available, by undoing the small nut on the upper (normally visible) side of the larger diameter part of the unit and sliding it one way or the other, depending how you want to adjust it - sliding away from the bi-metal spring housing (the tube with the electrical connection on top) will give a slighter higher speed when cold. If that doesn't fix it you'll probably need another one. I've pulled them apart and made 'em work but its a fiddly job.

If the idle mixture isn't the same whether the engine's hot or cold, then that's probably the control pressure regulator. They can be taken apart and (usually) got working again. The main problem with them is corrosion of the tiny plunger which bears against the stainless steel diaphragm which regulates the control pressure. Its pretty straightforward to overhaul. Make sure everything in the actual regulator is spotlessly clean, the small o-ring inside is ok, and clean and grease the plunger and you'll probably find it'll work ok. If it doesn't, it is possible to mess with the spring inside(pack it with a thin washer to get a leaner idle mixture or grind a bit of each end for a richer mixture). It doesn't take much change to the spring inside the main body of the unit to have an effect.