Thanks so much for the above suggestions
We first tried to see if, when we unplugged all output wires, the voltage would drop to zero then the left handle went down. That didn't change anything and the problem still persisted
We're a long way away from any MTH service centre and didn't want to buy a new Z4000 without first seeing if maybe the left side gear alignment was the culprit
So we crossed our fingers and opened up the case abiding by the cautionary suggestions gleaned (thankfully) from the Forum members above. The 7 screws came out ok (only hand tightened) and the cover was able to sit gently on the base of the case/handles.
The black marks on the white plastic gears were aligned on both the offending left side and on the right side so no need to re-align.
However we immediately saw that one of the clips holding the potentiometer (the round brass cased unit attached to the small gear) to its board on the left handle was slightly open compared to the clips on the right side. The potentiometer looked skewed compared to its twin on the right
We used needle nosed pliers to bend the offending clip so it was as tight as the right side.
Put the case back together, powered up, tried raising the left and right handles - low and behold the voltage now goes to zero when the left handle is lowered !
Thanks sincerely to Vernon Barry, Scouting Dad, Alan Mancus and gunrunnerjohn who guided us !!
Stew