I think i've almost got this figured out now ... it's a bit tricky, but it does what I need it to do. So i'm posting it here for others to use if they need it.
OK, so there is a main variable in the game called
killer countdown, this is set to zero in the main game loop (as per this screenshot)
On the map there is an event called a spawn point that when walked over sets a random number between 0 and 10, if the number is a 10 it turns on its self switch A and then sets the killer countdown to a random amount. It also moves itself to the location where the zombie pops up from.
On the spawn points second page, it looks like this ...
Obviously the event doesn't reset itself (if someone could suggest a method for this it would be great). But for now, it works and my brain can rest a little easier
So basically what happens is that if the player walks over the spawn point, it creates a random number. If that random number is a 10 then it triggers the spawn of the zombie, the spawn point moves itself to where the zombie will appear, and sets the killer countdown to a random number between 1 and 10.
The main game loop then picks this up and begins subtracting 1 from the killer countdown every second (or so) until it reaches zero.
Once the killer countdown reaches zero it no longer satisfies the second page of the spawn event and as such "turns off".
