Ever since I have had the car she has happily idled at 1000 rpm. I wasn't worried, but did wonder why everything I read said that the engine should normally idle at 850rpm. But now I know how to reprogram the ECU I sent the appropriate code to drop it by 150 rpm. It seems to have worked well and when the engine is warm it happily quietly burbles along at 850rpm with no hunting or surging.
What I found during the investigation is that the Rover engines normally leave the factory with a default idle speed set at 950 rpm but with a tuneable offset of -100 rpm. For some reason the (my?) Superspec seems to have left the factory with the offset at +50 rpm.