Hi Richard
I've made a bit of progress thanks to your help as I now have an icon on the main screen palette which turns the currently selected notehead into a cross when clicked.
I'm in a better place for drum score editing now than the default config but its a shame that bit of scripting / the new icon only works on the currently selected note and not multiple notes / the current selection.
There's not some special way I should be selecting notes is there? I've been LMB dragging over the notes I want to adjust, they get highlighted by dark yellow/green columns then I push my new shortcut icon and that only changes the note under the denemo cursor.