On the title screen, type "CHECKDISK" and it will check the program disk for errors.
During play, hold down "ZXCUIO" and the screen will flicker. Make sure you press them in the given order. Now use the following keys:
'F5' ------------------ To add extra lives
'F6' -- Subtract from current stage number
'F7' --------- Add to current stage number
'F8' ------------ Go to the selected stage
'F9' ------------------- Go back one stage
'F10' ------------------ Advance one stage
The up and right cursor keys increase something by one and the down and left decrease something by one.