Thank you for your help and suggestions.

Johnathon; you are correct and I have gone back to using food for the more sedate exercises such as the stays and the stand. But heeling is one of those exercises that you need a high level of accuracy and focus. The downs and the sits also need to be fast and reliable (not half cocked in case you get the ball. . . .)
Ann-Marie, I don't have sufficient privileges to open those links
I can see that I need to establish some rules in relation to the reward itself! I think I will do some work with the ball and a simple command and go from there. BTW, Kira does know the commands heel, sit, down, stay and now stand but I haven't worked out a suitable release command. What do you use?? Also, as far as returning with the ball, should I really go and get the ball off her or should she bring it to me??