Hi there LifeInAsia,
I can't explain it then; I agree with you that 'Primary' index (with a key symbol next to it) is by definition singular & not plural - but my MySql GUI tells me that under indexes, I have two Primary Keys, I wish that I could show you a screengrab of this, but I can't.. Oh well.
Well I pleased that I got something correct! And thanks for the Venn pointer, I never realised the connection before, and it's be a LONG while since I did that at school.
As I say, I'll ponder this a little more on Monday, I'm not really on any time constraints for completing this project now. Just when a manager says 'Can you just...', it ususally means a lot of un-necessary work that will probably be forgotten by the time he wanders off!
Thank you for the continued help anyway,