http://en.wikipedia.org/wiki/Paul_Leonard-Morgan
Paul Leonard-Morgan is a BAFTA award-winning and Ivor Novello nominated composer.
Apparently he's worked with a considerable number of Scottish bands over the last few years, as well as scoring numerous TV and film projects.
Update: http://twitter.com/mogwaiband/statuses/13142880777:
mixing The Singing Mountain
So presumably this is the new track with loads of orchestration, hence the composer/arranger?