Being both lazy and easily confused, I thought I would try 34theletteretc's simple suggestion first and, disconnecting my mimic panel with stud and probe point control and indicator LEDs from the 16V AC socket, all is well! I shall now power the panel etc from a spare 'The Engine Driver' dual controller with 16V AC outlet which is probably as old as my H&M Duette. Having tested it, everything works.
Many thanks to all for suggestions, I knew there would be someone here with an answer. I shan't ask anyone to explain what the issue was since I shan't understand.
I fear I may come back as other problems rear their ugly heads.