Ok, here's a little bit of a guide after some experimentation today. It might not work for every car, but it has certainly worked for Nicole...
This is how I've got her idling back at a normal speed within the space of an afternoon.
1445 - 64mm TB fitted. Idling at ~3,750RPM on first start, left to run for 5 Minutes.
1455 - Started back up. Idling at ~1,750RPM. Left to run for 5 Minutes.
1505 - Short 10 Mintue drive keeping below 3,000RPM. Idling when stationary at junctions at ~2,250RPM.
1545 - Started back up. Idling at ~1,500RPM. Left to run for 5 Minutes.
1645 - Started back up. Idling at ~1,500RPM. Left to run for 5 Minutes.
1745 - Started back up. Idling at ~1,250RPM. Dropped to ~1,000RPM within 1 Minute. Left to idle at ~1,000RPM for a further 4 Minutes.
The proof in the pudding will come later on when I do the Tesco run, hopefully the idle will stay at ~1,000RPM now.