Sorry jdsf, I think I might have missed your post for some reason. Thx for the suggestions - next time I come across some nettles (I recently threw some away - drats), I'll try your idea.
But to answer your ?'n - no, there's no cactus nearby. Or anywhere else in our garden for that matter. We used to have an indoor winter-flowering cactus (I think it's called a Christmas Cactus), but even that's gone now. It was making too much of a sap mess everywhere around it.
Anyway - UPDATE!!!
I think I may have cracked it. I was just checking my Gk Black when I noticed what looked like a small twig/leaf, about .25" in length and a sort of terracotta color. Just as I approached it with a finger to flick it off, it took off at terrific speed.
It was alive!!! What's more, there was leaf damage at the exact spot where it was sitting. This has gotta be the prime suspect, & if I knew it was some kinda insect, I woudda snapped a 'photo for the entomologists here to identify it for me. At least I now know what I'm looking for, & next time I see one taking a meal break on my beloved Gk Black, it's gonna go into the 'photo album.
EDIT: One of the shoots has died back, possibly for lack of foliage?