http://www.netrover.com/~timt/cheats/m/madprofessormariarti.html (Amiga Plus Extra No. 5/97, 05/1997)Wait for the demo to start and type "PROFMANIAC". The screen will flash letting you know it worked. This will give you infinite lives and the ability to start at any level.
NOTE: The demo starts right after the highscore table is displayed.