Richard Rodgers and Oscar Hammerstein wrote a song for a musical. Many years later a chap called Gerry Marsden recorded it with his beat group. Football fans in Gerry's home town subsequently (or according to M Carthy, previously!) started singing some of the song on the terraces. Supporters of other football teams started singing it, sometimes with altered words. Eventually an enormous crowd sang the same song to the Queen in front of Buckingham Palace.

I refer of course to "You'll never Walk Alone". We know it came from a musical called "Carousel" - we can buy the sheet music - and yet it would appear to qualify as a folk song, because most of the singers probably have no idea of its source.