Trying to build a Full Width Page in 2011

I’ve created a copy of page.php and renamed it full-width-page.php. I’ve added an id to the main content div that I can style to extend the page width from the normal template value. I’ve selected the Full Width Page option in the template selection box.

<?php
/**
 Template Name: Full Width
 */
get_header(); ?>
	<div id="primary">
		<div id="content-fullwidth" role="main">
			<?php while ( have_posts() ) : the_post(); ?>
				<?php get_template_part( 'content', 'page' ); ?>
				<?php comments_template( '', true ); ?>
				<?php endwhile; // end of the loop. ?>
			</div><!-- #content -->
		</div><!-- #primary -->
<?php get_footer(); ?>

I’ve styled content-fullwidth

#content-fullwidth {
	margin: 0 1% 0 1%;
	width: 98%;
}

The url to the page is https://www.marionandpeter.com/group200/?page_id=107

Despite all this the page seems to display unchanged. Any pointers appreciated.