Postscript to incremental design
Servos can be a pain! The Down Home worked fine with the new signal wires (actually a new servo with its original, non-extended servo wires) for the KoS locos (one Portescap, three Mashimas and a Bachmann motor), but not with my old Triang-motored diesel shunter on the Up line. The signal arm did not twitch, but simply moved from Off to On, and beyond, as the loco passed through the area around the switched common crossing (SCC) on the loop point.
I reasoned that the problem was that the signal servo wires were virtually surrounded by common return wires, and when the point was set to Up, the SCC was also set to common return. When I re-routed the wires looped around three sides of the servo board and moved the servo board further away from the track, the loco did not affect the signal.
I am claiming this as some sort of success, but still find it irksome to have to go through these hoops (loops?). However, it’s given me the confidence to continue with servo-operated signals, and to finish this round of wiring mods. The Up Starter will have to wait for me to produce another servo board: it’s too far to plug into the Down Home board.