I realise I may be talking to myself here, but in case anyone is interested, I finally heard back from the iBookstore on this one. Yes really, I did. They said:
"For changes that are cosmetic we currently send push updates when there is an ePub not functioning properly but do not send push updates when an ePub is functioning as it should. We value your concern and will take this into consideration for future modifications.
Best Regards,
The iBookstore Support Team"
So the fact that I have discovered how to make all the bridge books I've published look much nicer by being able to include unicode characters for the card symbols and made all the necessary changes and uploaded to the iBookstore still means all prior purchasers won't get any updated version unless they happen to know to delete their copy and redownload.
IMHO you can have as much technology as you want Apple but unless you have customer-centric processes to match you are failing.
Rant over. I may even give myself a correct answer.🙂