IanC: very interesting set of posts, plenty of food for thought. I see that you don't really go into one very vexatious about the age of a specific song: how you define the song before you try to date it. For example, we often read in fossil/evolution stuff, of things like "the earliest human". Since we all go back one generation at a time to the first amoeba, youve first got to decide PRECISELY what you mean by "human being" before you can try and date when the first one arrived. I know you are well aware of this, but it does need to be dragged into the discussion at an early stage. Because of the evolutionary nature of a folksong, we nedd always to know just what mean by "Dido, Bendigo" before we start on the possible dates.
    On that specific topic, because the word "Bendigo" may have been inserted into an existing song because of the current fame of the Bendigo, I think we should take a look at the "Dido". I think this points to the song not being much before 1000BC.