Scripts triggered via advice, "like" show_me

[Makanyane]

The siege scroll as a trigger sounded such a good idea I thought I'd try it so have been trying :

monitor_event ScrollOpened ScrollOpened seige_scroll
and I_FactionBesieged vandals
console_command create_unit Metatron "semin" 1
console_command create_unit Metatron "semin" 1
end_monitor

that's simplified version to test it as I can't get it working, the create unit bits worked with End Turn event and settlement under siege conditions, but switching to ScrollOpened kills it - any idea what I'm missing?

EDIT: I can get it to work on buttons eg;
monitor_event ButtonPressed ButtonPressed siege_assault_button
though I can't see anyway of stopping it triggering for other cities that meet the same faction ownership requirements and have enemy army standing near them (set on counter) - though I suppose if they have enemy army standing near them....