Just realised I was being thick, reason it CTD's if slave faction is at top of strat is because CTD causing error (rather than other ones) is in slave section.

You've got
character, sub_faction parthia, Aaron,
for the slave faction - Aaron is from slave section of descr_names, which is unfortunately misleading as you don't actually use those here.

If you have sub_faction parthia - then you have to have name from the Parthia section!! Having name from the wrong factions name list is one of the causes of errorless CTD's.

(Am double posting rather than editing in hope morano spots additional info.)