Knowing the habits of modern day filkers, and having rewritten many songs myself, I know that songs can be altered dramatically with no intent to suppress the original version. I also know that songs that start clean can acquire extra verses that are decidedly unclean.
As Iles's entire approach is based on the premise that the songs were all originally sexual/pagan in content and that this was edited out and changed by clergy or song collectors, I have to disagree with most of his book.
I could easily believe that many traditional songs started clean, acquired extra smutty verses that were sung in the apropriate places and that these verses (sadly) were often not passed onto collectors. This does not, however, invalidate the collected song.
(Iles's approach is a bit like assuming that "While shepherds washed their socks by night" must be the original version because the church would clearly want to suppress it.)
The whole book is even more frustrating because now and then he makes a case that does stand up to inspection. It's just that you have to wade through too much supposition and conjecture to get to the useful bits.
Rarely do I shout out loud in protest while reading a book - this one had my family getting inundated by irate comments.
I had to go away and read some Ronald Hutton to remind myself what research into folk traditions really means.
If anyone wants this book, just ask (but you'll have to remove Hutton from my cold, dead hands)...