Once the game has loaded, press the 'SPACE BAR' to view the options
screen.  Type "OPERATION FERRET" (including the space) and the screen
will fade to black.  The player controls will now say "Player 1 is on
acid" indicating the cheat is active.  Start the game and you will
have infinite lives!

For a hidden message from the programmer Richard Aplin, open up the
file s:startup-sequence.