Trying to create custom actions / filters – help needed!

Hi all,
I am adding a sidebar into the single-post view of the Kubrik theme (done successfully) and I would like to create a custom action / filter tag that the theme would pick up on the sidebar I’ve created. From what I’ve read of the WordPress source code it should be a case of simply adding the filter via a plugin, then calling the filter-functions by using the “do_action” function.
So far I have:

\wp-content\themes\default\sidebar2.php

<div id="sidebar">
<ul><li>test include sidebar</li>
<li><?php do_action('single_sidebar'); ?>
</ul>
</div>

And a plugin to test the action:
\wp-content\plugins\single-sidebar\single-sidebar.php

<?php
/*
Plugin Name: Sidebar 2 Hook Test
Plugin URI: https://www.tamingentertainment.co.uk/
Description: test plugin
Version: 1.0
Author: Toby Berriman
Author URI: https://www.tamingentertainment.co.uk/

Copyright 2008 Toby Berriman
Released under the terms of the GNU GPL v2

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

*/

function ss2() {
	$content = '<p><em>Test action worked!!!</em></p>';
	echo $content;
}

function remove_ss2() {
 remove_action('single_sidebar', 'ss2');
}

  add_action('single_sidebar', 'ss2);
   register_deactivation_hook( __FILE__, 'remove_ss2' );
?>

Unfortunately although the plugin activates successfully I am not getting the extra text on the sidebar that I expected ??

Before anyone asks, the reason I want the custom action/filter is so that I can easily move plugin content that is applied to ‘the_content’ into the sidebar ??

Any ideas / suggestion?
~Shiv