The poem itself is in loads of places on the internet, sometimes credited to Doreen Valiente, sometimes not (search on "Witches Ballad", with the quotes). It seems clear though, that it appeared in her book "Witchcraft for Today", which I'm sure I have somewhere, but I can't find it just this minute.

Having found Castalia's website, I didn't search any further for a tune. But of course, it's not beyond the bounds of possibility that others have written tunes for it too. So I've searched further, and found a mention on this forum - a mother who says her daughter wrote a tune for it and sometimes sings it. The daughter, she says, is in a band called Blaming Grace , but neither their main website nor their myspace site have any mention or recording of that particular song. And it's unclear which of the band members is her daughter!

Not very helpful, I'm afraid, but will carry on looking.

Joan