On the options screen, type "SCRAP 28" for infinite lives. Press '0'-'1' on the keypad to select levels. Use the function keys for variable slow motion.
On early versions hold "HELP" and fire together, with the above keys to activate the cheat.
If you have the latest version then attempting the above will reveal the message: 'Congratulations!, you have found the cheat mode... unfortunately it has been ZZAPPED!'. If this happens, go to the control selection 'F10' and type "scrap 28" and then start the game. The effect is the same as above.