It still doesn't make any sense.

Whatever I try I get :

XML Parsing Error: not well-formed
Location: http://yoog.co.uk/new-playlistpage/
Line Number 101, Column 4:

Or something very similar.

OK I have got what I needed now.

PlaylistPage.php:

<?php
/**
 * Defines the Playlist page type
 */
 
class PlayListPage extends Page {
   static $db = array(
   );
   static $has_one = array(
   );
   
   function getAList() { return "something returns but this is just a string for testing";}
   function Content() { $content = $this->Content; $content = str_replace('[[aList]]', $this->getAList(), $content); return $content; }

}
 
class PlayListPage_Controller extends Page_Controller {

}
?>

Now I can put [[aList]] in the content box in the CMS and I get the test text comes back in the editor. This is what I needed. I have to think about my editors. :)

Thanks for your help guys.

XML Parsing Error: not well-formed
Location: http://yoog.co.uk/new-playlistpage/
Line Number 101, Column 4: 

Watch out for that. The default theme sends the html as pure xml which is picky. You can disable it (and avoid errors like this) http://doc.silverstripe.com/doku.php?id=contentnegotiator

It seems so simple once it's done. I suppose it's just getting used to a different way of doing things!

Thanks again.