There used to be a bloke called Gordon Ridgewell - quite possibly still alive (check letters to English Dance and Song: the ones that end Wassail instead of Yours sincerely are from him!) He had a theory (some might say an obsession) that all the surviving traditional carols were originally pagan and that the church (overtly or covertly) forced people to Christianize them. So far so good as a theory - might even be true (in at least a few cases). The problem is that he then tried - on the basis of very little evidence as far as I can see - to reconstruct the originals. He used to sell Christmas cards with the words on. In my opinion they are unsingable, but who knows ... I'm afraid the few examples I had, having been kept since the 70s, finally got the order of the boot when I moved last year. But someone may still have some; and there was a book on the same subject, I think.