Hi all, thanks for your replies - sorry for the slow response, had a bit of a busy couple of weeks. That was a very interesting document Jessopia74 - after following the instructions the car seems to be idling nicely again although I only had time for a quick test run today. I'm not sure if I'm fully out of the woods - it idled at 1000 rpm all the way from cold start to hot, except for when I stopped at one particular roundabout and it was nearer 1500 rpm - but oddly that only happened once.
I think maybe I need to use some of the throttle cleaner on the throttle mechanism itself - it could be that it's not always returning fully to the idle position, and the matter of 1/2mm makes quite a difference.