http://www.netrover.com/~timt/cheats/d/dragonslair.html (Amiga Plus Extra No. 5/97, 05/1997)Hold down 'ESC', 'R', '/', 'L', 'N', and '7'. Now press the fire button to watch the demo of the entire game.
Another one is to hold down 'ESC', 'RIGHT' and 'LEFT' cursor keys, 'N' and '7', then press fire.