Thanks for your reply. I did a bit more considering given what you suggested, and that has led me to find this line:
{Keys::Delete, SCI_SHIFT, Message::Cut}
in /scintilla/src/KeyMap.cxx. I think that means that the shortcut is one of the defaults for the Scintilla component, and... then I'm guessing... that any of these that Geany does not explicitly override are present in the editor.
I can't see any way to 'mute' that shortcut, then, without a recompile, but I've settled for assigning that shortcut for something as innocuous as I can find (it'd be great to be able to map it to 'DELETE' but that seems to not be possible). This works well enough.
Thanks again for the help,
Best wishes, Ben