More farting about later, I now have a Swamp class which has a collection to hold creatures. I can add an ogre to a random location, then decide (and for now, report) whether an enemy is to be added, then do any necessary enemy-population augmentation. (The enemies always arrive at (0,0)). I also have a method to report which creatures are currently in the swamp and where they are.

So:

1. Add an ogre
   
2. Will an enemy arrive?
   
3. Yes – here it is!
   

On the next go:

1. Add an ogre
   
2. Will an enemy arrive?
   
3. No – poor ogre is lonely (but he likes it that way).