It's "The Lonliest Road" by The Books
It was used on a radio play written by Gregory Whitehead but the music was commissioned specially for it. A quick Google suggests that it can't be bought :-( (I've not checked iTunes or Spotify though...) but you might be able to 'acquire' the whole play from one of the manyfilesharing sites out there.
Good luck in finding it, I love the feeling of being able to listen to a full song when one or two lines have been bugging me for ages!!