Oh! I love this! The Doctor and Donna have absolutely perfect voices and made me laugh with joy, and it's right and just that Cordelia becomes a companion at the end. She so needs something for herself, although I suspect she'll also see the universe as her duty to save.

And oh damn. I just reread it and the hurty bit's at the front:
It was the song of freedom, of escape. Donna thought she would remember it for the rest of her life.