Newsnow.com ask that articles submitted to their news service have the
comment added to the html as below. How can I make the stories
published on my joomla site include these tags at the correct place?

Here are the newsnow.com rules:

To ensure we can accurately process the text of each article you
publish, we require that you include the following comment tags within
the text of your article pages:

   1.

      <!-- Article Start -->

      should follow the headline but immediately precede the first
paragraph of the article
   2.

      <!-- Article End -->

      should immediately follow the last paragraph of the article.
   3. If your articles contain a byline or specify an author:

          <!-- Author Start -->

          should immediately precede the author's name;

          <!-- Author End -->

          should immediately follow it.