Update (for my own peace of mind). Frameset is now with Argos Cycles in Bristol for a re-finish to flamboyant red, and new steerer tube so I have more options. Lovely shiny Cane Creek Classic 100 Threaded Headset waiting to be fitted. Email traffic under way with Edward Rogers of Chris King (as of 18 Jan) so I live in hope of finally understanding the problem and whatever I find out will be posted here for others' delight/to avoid the mistakes I made.