The message
from "mbb" contains these words:

Is the answer to have a posting, plus replies, each of which only contains
it's own text?

No, because other users may miss the original post. It's usual for
threads to have multiple participants coming and going at different
times and broadening the original topic. Posters need to indicate which
later contribution they are responding to.

(snip)
Sorry to go off-topic - if anyone can point us to authorative Newsgroup
etiquette sources, that would be appreciated.

Don't worry, your question is very welcome.

A short version of "how to edit a reply" can be found in a help post
that appears here every week, called abc for newcomers. That post is
also permanently located in google's archives of this group, search at
http://groups.google.com/ (easy help available there).

http://www.greenend.org.uk/rjk/2000/06/14/quoting

is an excellent guide to how to compose and edit replies. For general
netiquette

http://groups.google.com/googlegroups/help.html

Janet.