Thanks a million it works.
Could I possibly pick your brains with a further question.
There is also the possibility of insufficient enrollment in a class, in which case it will not run. If the minimum number is 6, I have used the count summary field for a calculation that returns "Insufficient Enrollment" if there are 5 or less students enrolled.
This works up to a point, but the problem is that it does not change after the minimum number is exceeded.
If you are busy I post this to the forum, and I promise I won't ask any more questions.