Could any mod's tell me which is the default option - threads sorted by time of last post / sorted by first post.

I got the impression that the latter was the default, and I think this may not be the best way to sort threads (by default), since people may be missing long discussions that started several days or weeks ago, and those who haven't changed it manually to sorted by last post will not be seeing these threads on the first page of a forum even though discussion on them is raging.

If the default is 'sort by time of last post' - ignore all of that

Consider it ignored!