What is the best way to add a comment policy to the comment form? Surely this is a common use-case... I would have thought? Underneath "Add new comment" heading I would like to add a block of text like "Comments are pre-moderated and may be edited before publication". I have an internationalised site so the text must be translatable.
You can do this easily with a form_alter hook.

&lt;?php
function MODULENAME_form_comment_node_NODE_TYPE_form_alter(&amp;$form, $form_state, $form_id) {
    //drupal_set_message('&lt;pre&gt;'.print_r($form, 1).'&lt;/pre&gt;');
    $form['privacy-note'] = array(
          '#markup' =&gt; t('Comments are pre-moderated and may be edited before publication');

        );
}


Yes it sounds a little silly to make a module for this. But making a little module for every little work is also not a smart option. 
If you have never created a module before, See this: http://drupal.org/node/1074360
Don't forget to change MODULENAME and NODE_TYPE with actual values. if you have any questions, I'd be happy to answer them here :)