Trouble getting alternating comment styles to work

Hi

I’m trying to get alternating comment styles working on my blog. You can see an example of a post here:

https://www.urbaneblinds.co.uk/blinds-blog/?p=3#comment-3

The problem seems to be that the class ‘alt’ is not being output into the code for some reason. Here’s what I’ve got in comments.php:

<?php // Do not delete these lines
 if ('comments.php' == basename($_SERVER['SCRIPT_FILENAME']))
  die ('Please do not load this page directly. Thanks!');

        if (!empty($post->post_password)) { // if there's a password
            if ($_COOKIE['wp-postpass_'.$cookiehash] != $post->post_password) {  // and it doesn't match the cookie
    ?>

    <p class="nocomments"><?php _e("This post is password protected. Enter the password to view comments."); ?><p>

    <?php
    return;
            }
        }

  /* This variable is for alternating comment background */
  $oddcomment = "alt";
?>

<!-- You can start editing here. -->

<?php if ($comments) : ?>
 <a name="comments"></a><h2><?php comments_number('No Responses','One Response','% Responses' );?></h2>

<ol class="commentlist">
<?php foreach ($comments as $comment) : ?>
<li <?php echo $oddcomment; ?>id="comment-<?php comment_ID() ?>">
<cite>
<span class="author"><?php comment_author_link() ?></span><br /><span class="time"><?php comment_time() ?></span> on <a href="#comment-<?php comment_ID() ?>" title=""><?php comment_date('F jS, Y') ?></a> <?php edit_comment_link('edit','&nbsp;&nbsp;',''); ?>
</cite>
<div class="commenttext"><?php comment_text() ?></div>
<?php if ($comment->comment_approved == '0') : ?>
Your comment is awaiting moderation.
<?php endif; ?>
</li>
<?php
/* Changes every other comment to a different class */
$oddcomment = ( empty( $oddcomment ) ) ? 'class="alt" ' : '';
?>
<?php endforeach; /* end for each comment */ ?>
</ol>

 <?php else : // this is displayed if there are no comments so far ?>

  <?php if ('open' == $post-> comment_status) : ?>
  <!-- If comments are open, but there are no comments. -->

  <?php else : // comments are closed ?>
  <!-- If comments are closed. -->
  <p class="nocomments">Comments are closed for this post.</p>

 <?php endif; ?>
<?php endif; ?>

<?php if ('open' == $post-> comment_status) : ?>

<a name="respond"></a><h3>Leave a Comment</h3>
<form action="<?php echo get_settings('siteurl'); ?>/wp-comments-post.php" method="post" id="commentform">

<p><input type="text" name="author" id="author" class="styled" value="<?php echo $comment_author; ?>" size="22" tabindex="1" />
<input type="hidden" name="comment_post_ID" value="<?php echo $id; ?>" />
<input type="hidden" name="redirect_to" value="<?php echo htmlspecialchars($_SERVER["REQUEST_URI"]); ?>" />
<label for="author"><small>Name</small></label></p>

<p><input type="text" name="email" id="email" value="<?php echo $comment_author_email; ?>" size="22" tabindex="2" />
<label for="email"><small>Mail (will not be published)</small></label></p>

<p><input type="text" name="url" id="url" value="<?php echo $comment_author_url; ?>" size="22" tabindex="3" />
<label for="url"><small>Website</small></label></p>

<!--<p><small><strong>XHTML:</strong> You can use these tags: <?php echo allowed_tags(); ?></small></p>-->

<p><textarea name="comment" id="comment" cols="100%" rows="10" tabindex="4"></textarea></p>

<?php if ('none' != get_settings("comment_moderation")) { ?>
 <p><small><strong>Please note:</strong> Comment moderation is enabled and may delay your comment. There is no need to resubmit your comment.</small></p>
<?php } ?>

<p><input name="submit" type="submit" id="submit" tabindex="5" value="Submit Comment" /></p>

</form>

<?php // if you delete this the sky will fall on your head
endif; ?>

Any ideas what I’m doing wrong?

Thanks! ??