Query not retreiving custom field post

Hi,

I have a query that is set up to display a featured post, to be retrieved by way of a custom field associated with the particular post. From what I have I can’t see anything wrong and I’ve been looking at it all too long now to perhaps notice something I may have overlooked? The query looks like this:

<?php
     $featured = new WP_Query();
     $featured -> query('showposts=1');
     while($featured -> have_posts()) : $featured -> the_post();

                        $wp_query->in_the_loop = true;
                        $featured_ID = $post->ID;
      ?>

      <?php if (get_post_meta($post->ID, 'preview', true)) { ?>

       <div class="image_block">
       <img src="<?php echo get_post_meta($post->ID, 'preview', true); ?>" width="230" height="160" alt="<?php the_title(); ?>" title="<?php the_title(); ?>" />
        </div>

        <?php } ?>
        <div class="text_block">
        <h2><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></h2>

        <small>posted <?php the_time('jS M Y'); ?> in <?php the_category(', '); ?> tagged <?php the_tags(''); ?></small>

        <?php the_content('Read On?'); ?>
        </div>

        <?php endwhile; ?>

With a custom field ‘preview’ associated with only the one post I wish to feature, the value of which is a url for the image to be used with the content of the featured post, should this not work?

If I put an else statement in to display the string ‘No posts match your criteria’ the query returns this string, rather than the post. If I do not put in the else statement the query returns the latest post, but still NOT the post with the custom field.

Is there something I’m doing obviously wrong?

Thanks.