Thanks for confirming my suspicions about the LED. Glad to know my brain is still mostly working...
Hmmm. I might not have been sufficiently clear in my description of the power cable setup.
The cigarette-plug cable runs from the power station to the distribution box, then I use 2.1mm DC power sockets/plugs on short cables (no more than 75cm long) to power the equipment.
I was measuring the output voltage at the end of the SW cable, before it went into the distribution box, so it was the sole cause of the drop.
It's all academic now, as the new cable has zero voltage drop when measured at the same point.
Cheers,
V