Having extended the new system and carefully checking the wiring (including a test run) moved onto the next stage of cutting the track in way of the access flap. Fault it screamed (in several languages).
Like a good engineer retraced the new circuitry but even with total isolation failed to lift the fault. After two days of breaking down the system to it's furthest dark corner (just into Africa) that fault just disappeared! This left the new work to be recoupled together with an auto reverse unit. To the bitter end it played up -- right up until I lost it and batted it with my old navy green wand (for electrical things) and screamed at it with both split infinitives, invective and foul curses.
It is working fine thanks.
Lesson: DC wiring techniques have a lot to commend them in a DCC world.