Thank you both for your replies. What you say makes sense.
To answer your questions...
Three aircraft, KX, OR, WZ;
KX & OR - Classic
WZ - a hybrid. Pi B3, with 2 x NooElec SMArTee receivers
KX & WZ, power from certified USB socket, TA102. OR via Anker 12v to USB cigar lighter adapter.
Swapped the receivers in KX & OR for the new NooElec, as advised. Some success, but still issues, see the screenshots below. after the swaps.
KX - was the image displayed at start of this chain. Forgot to take screen grab today, but was four greens for the devices but still ERROR on the voltage.
WZ:
OR:
The power cable on OR is the better of the three. It's thick, short, hard-wired to the PAW via a toggle switch (see image). KX is a short flimsy cable with USBs both ends. WZ is a reasonable cable, hard-wired at the PAW end but is 1.5m long and I wonder what the voltage drop is in it. Will replace both KX & WZ cables in due course.