I should imagine that nobody mentioned the Wren Song because there is no evidence that there is anything "pagan" about it.  "Virtually unwritten" sources are a bit of a problem, really, since by definition it's only possible to have an opinion about them once they exist in recorded form.  A song that was observed and written down 50 years ago may have been around for "several centuries", but how do you know?  It could just as easily have been made up 51 years ago.  Unless we really want to look like the silly stereotypes that people who despise folkmusic would like us to be, we need to be extremely careful about what we claim; and we need to be able to back up our claims with evidence.

Malcolm