To answer my own question, there is a wonderful collection of old timetables online, and my sister "Jeniffer" (as I referred to her) recalls it was July 1974.
http://www.timetableimages.com/ttima...4/da744-08.jpg
So I even have a flight number for the first leg, DA391. Most exciting, that's certainly going on myflightradar24 when I've found the rest of the trip...