Moj you are a star!! I have just dusted off my chewed Perfect puppy book and found the exercise you describe on p104/5.
I'll type it up in case Poppington hasn't got the book!
"Exercise: Elementary chase recall
1) You will need two people and two toys. Begin by throwing a toy past your partner, who should be standing facing you some distance away. Do this several times and allow your puppy to chase after the toy and bring it back to you.
2) About once in every ten throws, at random, throw the toy towards your partner, who should catch it and tuck it away out of sight. As your puppy begins to run after the toy, shout 'Leave!' loudly. Since the toy he was chasing has effectively disappeared, he will eventually look back at you to see if you have it.
When this happens, tease him with the other toy and throw it in the opposite direction. Allow him to chase and retrieve this one, but try to make this chase more exciting, either by pretending to go after it yourself or by using a more interesting toy.
Eventually your puppy will learn that when he hears the voice-cue 'Leave', there is no point in continuing with the chase because the more rewarding chase game is now back with you"
It also says that it may take several weeks to get, shouting 'Leave' to often will result in hesitation before running to get toy so exercise won't work! It also says will need to increase the exciting circumstances you play this game.
I can get Luna to STOP when chasing a toy. She will also wait before going to fetch a toy that has been thrown. I guess I need to be more exciting than the toy after she has STOPPED (shall take favourite footie toy tomorrow as well as tennis ball!)
Worth a try!
edited to say! Typed too slowly! Cross posted! This book was my bible for so long! Can't believe I missed out on that bit of training when Loon was smaller!!!!!