The Oxford Book of Carols has quite a lot of Carols in several versions and with alternative tunes.

Well, Vaughan Williams was one of the editors, so you'd expect it to contain lots of folk songs, which it does. It's got a splendid preface by his fellow editor Percy Dearmer (the third one was Martin Shaw) outlining the history of Carols, and how the traditional Folk Carols struggled to survive in face of clerical disapproval - very much the same story as the opposition to West Gallery Music in the same period.

I couldn't find the Preface on the net, but it might be there somewhere. If Tim Radford, who started this thread, could read it I think he'd find it useful. So would anyone else interested in Carols, which I suppose at this time of year means most of us.