After driving at a constant 100k's for a few miles with the revs around 2000rpm, when I pull up the revs stay there, until I flick off the ignition a couple of times, when it returns to about 700 idle.
When starting warm it also revs up briefly to about 2000 before settling back to idle.
Could this be a sticky IAC? I swapped one over from the spare car because it looks a lot cleaner than the old one. It was when I was chasing a non-starting problem that was resolved with a replacement crank angle sensor.
The cable to the throttle body is free and returning to it's stop as normal.
So the Idle Air Control seems a very likely suspect.