[youtube=http://www.youtube.com/watch?v=n2xODjbfYw8&w=700] Rather stupidly I just listened to this three times in a row and then had to go for a walk. SUCH a beautiful song. NOT something to listen to more than once. DEFINITELY a little bit of magic in there for all of us.
It was originally recorded by Sandy Denny privately in 1967, and again, only accompanied by herself on guitar (and a string section, both versions, with and without strings, were released by now), for the All Our Own Work recordings (with the Strawbs). It is best known in the version sung by Denny on the Fairport Convention album Unhalfbricking.
The guy who posted this version on YouTube says: "Fairport Convention were one of the most innovative and influential British bands of the late 1960's and are still recording and touring today. The lead singer during their greatest period was Sandy Denny who was in my opinion the greatest female vocalist of that or any era. The enormous emotional range of her voice has been unsurpassed by any other singer. Sandy died tragically in 1978 of a cerebral haemorrhage after falling down a flight of stairs."
ps - I just stumbled on this demo version, which is lovely: