Generating the Title of web page using a new string?

My site doesn’t have any posts, just pages, and a lot of pages have the same title so the <title> of each web page looks the same. Which is no good for SEO.
So I have created a new variable in work-meta-box.php that enables me to have a field called ‘seotitle’ when I create a new page. So far so good.

But how do I edit header.php to generate the new <title>?

Currently header.php says:

<title>
   <?php
 	global $page, $paged;
 	wp_title('|', true, 'right');
 	bloginfo('name');
 	$site_description = get_bloginfo('description', 'display');
	if ($site_description && (is_home() || is_front_page())) { echo " | $site_description"; }
 	if ( $paged >= 2 || $page >= 2 ) { echo ' | ' . sprintf('Page %s', max($paged, $page)); }
 	?>
</title>

Thanks. (I’m a newbie to php)