With David Tennant having taken his final journey as nation's favourite Time Lord, here are some more classic regeneration clips from the Doctor's previous incarnations. The 21st March 1981 saw Tom Baker's near-seven year reign as the Fourth Doctor come to an end when having fallen from the gantry of a radio telescope he merged with the mysterious Watcher. Watch more high quality videos on the new BBC Worldwide YouTube channel here: http://www.youtube.com/bbcworldwide