Thanks for your help, but it wasn’t the solution I was looking for.
I did find a fix though which I’m still working on.
Eventually I want a function that will recognise if the checkbox in the archive widget has been checked and then replace the standard widget select area width the code below.
<ul>
<?php
/**/
$years = $wpdb->get_col("SELECT DISTINCT YEAR(post_date)
FROM $wpdb->posts WHERE post_status = 'publish'
AND post_type = 'post' ORDER BY post_date DESC");
foreach($years as $year) :
?>
<li>
<label><?php echo $year; ?></label>
<select name="archive-dropdown" onchange="document.location.href=this.options[this.selectedIndex].value;">
<?php $months = $wpdb->get_col("SELECT DISTINCT MONTH(post_date)
FROM $wpdb->posts WHERE post_status = 'publish' AND post_type = 'post'
AND YEAR(post_date) = '".$year."' ORDER BY post_date DESC");
foreach($months as $month) :
?>
<option value="<?php echo get_month_link($year, $month); ?>">
<?php echo date( 'F', mktime(0, 0, 0, $month) );?>
</option>
<?php endforeach;?>
</select>
</li>
<?php endforeach; ?>
</ul>