In 1967 Delia is credited as the co-writer of the track Firebird with David Vorhaus and Brian Hodgson for the 1968 album An Electric Storm, though I gather that Delia actually wrote it s few years earlier and dug it out for the album.
The Performing Rights Society's list of works by Delia Ann Derbyshire has:
Title:Firebird Writer(s): Derbyshire, Delia Ann; Vorhaus, David Glyn Publisher: Island Music Ltd
2:43 on vinyl, 3:00 on CD.
- Released on the album An Electric Storm