I'm an idiot
Of course, it didn't occur to me that it was because I was CTRL-clicking the redbox caption. When I tried CTRL-clicking within the selection area itself it worked perfectly
Pressing the right-CTRL key on its own (while the focus was within the selection box) also worked, except since I have Highlight the pointer when you press Ctrl
set in Mouse Preferences
I get the concentric circles appearing in the capture. This isn't a problem with CTRL-Click as the capture triggers on the click, so by the time you let go of CTRL triggering the concentric circles the capture's already been done.
Maybe there could be an option to make the trigger pressing down, rather than letting go, of the CTRL key, or to add a minimum delay of a second or so when activating with a CTRL key?
Anyway, thanks for the fast reply Mouser - sorry it was such a dumb question