I could have sworn I saw a mass of spoil pouring from the interior of the Monument when the door was first opened in an earlier version of the first film (that footage is not shown).
That footage however is shown here - http://www.youtube.com/watch?v=LxkXdK2hcs4 (04:40 minutes in). It's difficult to reconcile that footage with what seems to be a 'cleaned up' version of the opening here - http://www.english-heritage.org.uk/about/multimedia-library/conservation-projects/ (Holes in The Hill: The Conservation Project Begins. 03:00 minutes in).Perhaps English Heritage would like to explain the difference?