Hi Philip
For all I know you may have read these already but the following links explain how to truncate your posts in Blogger and will lead you to lots of Blogger experts if you get stuck with it.
Blogger Help : How can I create expandable post summaries?
in the official Blogger help section and also at Beta Blogger For Dummies here
Dummies Guide to Google Blogger Beta: Expandable Post Summaries
As far as I can make out this will only work for posts made after you implement the changes
Mo.