I thought the teaser and the excerpt were two different things?
At the moment, whatever the default is, that's what I'm using. What I'm talking about is when I write a post, my feed (in a reader, like the Wizz Reader in Firefox) shows the title and the first few words of the post. That's the part that I'd like to be able to change sometimes, and choose another section of the post to display instead of the first few lines.
Because I've only just started, I'm not sure what passes that information to the readers and feeds. My blog address is http://www.freesitebuilder.co.uk/WordPress/ and I'm using Feedburner for the feed.