Thanks (partly) to Anthony Britt's help I've now resolved the idling problem on the Cavalier.
Anthony suggested adjusting the TPS to give 0.57v at idle. It was at 0.62v before I adjusted it, so I set it to 0.57v but the idle speed was still too high. Eventually I turned the TPS to the lower limit of it's adjustment, resulting in a voltage of 0.51, the engine then started to idle correctly at about 900rpm. It is now much more pleasant to drive, with the revs dropping promptly to less than 1000rpm when the throttle pedal is released. The throttle is a bit less sensitive now too, not sure why, but it makes it a lot easier to control at low speed.
Thanks again to everyone for their help.
James of Sunderland