Yes, the CSS is 'just' for a particular plugin. I thought there would be a way of calling it from the plugin itself. I know I can't use include() as this just prints the CSS to screen as opposed to applying it to the output from the plugin.
pas I said, I saw this was done for the 'tags' plugin but this is a very complex plugin. Mine is very simple and only outputs through the .tpl for the plugin, under plugin.php page. (This is a standalone plugin, I.e it doesn't hook or tag into any other parts of Cotonti, just onto the plugin page)