Fault found. Recently noted in an image recorded with a thermal imaging camera, that a connector for a lead to a large electric motor driven pump was shown as being very hot. It's possibly been like that for several decades, as the apparatus was manufactured circa 1970. Cable mounted connector was okay, despite having been about 50 degrees C on the outside, but the above wire pulled straight out of its clamping socket in the matching control box side of the connector. The heat generated at the only just touching contact was starting to melt the wire's PVC insulation. Had to cutback the wire a bit to clean copper, and then tinned it with solder. Cleaned up the matching screw clamp and terminal area to make a good contact, as that was also tarnished from all the heat. Tightened up, replaced everything, safety and operational test for over 30minutes were done the following day (this is a back-Blip), with no signs of the connector heating up now on that thermal imaging camera.

The type of wire they'd used all those years ago, is not ideal, with its few thick strands, but it would be a major job to replace all of it throughout the control box. Should be okay for our future usage. Don't know if it'll last another 50+ years!