Here is Crucible's page at Wildgoose Records. A further click will take you to a description of their album "Changeling." Note that Changeling is an album, not a song. It does, however, contain a song called CHANGELINGS (sic) LULLABY.

Unfortunately, that site has no samples, lyrics, or notes about the song—not even a writer's credit. (But a review says: "words Gavin, tune and singing Jess.")

Crucible's own web site has even less information.