Charles Nicholas at CN4x4s in Wales sent me another used door latch. In better external condition, it works perfectly.

After some forensic examination, the original latch has worn microswitches which make/break on their own, so their replacement would probably repair the lock mechanism. The initial latch from UK purchased to replace the original had issues with the rear lock switching which could also be easily repaired. Even though this vehicle had been relegated to a horse feed carrier on the stud near Rushworth, there was no sign of corrosion in the wiring, connectors or switch gear associated with the locking circuits - a big plus, I think.

So all's well that ends well, until the next time.

Thanks to all - the advice was helpful and to the point.

Cheers, all!

Pete