OMG how have I never seen this before?

Noooo, why can't I embed? Here's the link:
http://www.youtube.com/watch?v=3s4Czla6tXc&feature=player_embedded

(David Tennant and various members of the cast & crew singing “500 miles” by The Proclaimers.)

( Can someone tell me which episode this gif is from? )