Ok, since the dist cap, rotor, ht leads etc are all pretty new, and this was something I could do straight away, I did the idle air controller degunking thing, as per RollingThunder's suggestion (and thanks very much for the link to the picture essay), which went well enough.
I used carb cleaner as solvent. A lot of blackened fluid poured out, but eventually it came up shiny.
Time to test. Flat battery...
Later, took it for a short test drive, and since, on a few longer drives.
I think it's probably not tempting fate too much to say the issue appears cured.
In fact, it appears subtly smoother at any rev (is that likely, or is it just my wishful thinking?).
Thanks to everyone who offered advice and observations (relays,wiring, sensors etc. duly noted for future ref.): it's truly good to receive such support. Good on yer!