Another hare-brained idea

This was the reason I found

"sometimes the carbon rod will act as a parasitic element and thus modifying the antenna's radiation pattern"

but as I said - I moved the receiver back 1/4" - both aerials in same orientation and that cured the issue. Strange but true. Difficult to get the orientation wrong on a Spectrum AR635 receiver. The aerials are only a couple of inches long, are at right angles to each other and the receiver has specific positioning requirements because it is an AS3X jobby.

I have also read that ESCs can cause interference. I don't take any of this lightly and always range check.