Filter child pages by tag


I’m trying to create a simple foreach which displays the latest testimonial from the news page but for some reason the below isn’t working - Any help / advice as to what I’m doing wrong here would be much appreciated.

If I remove ‘->filterBy(‘testimonial’, ‘,’)’ then the content from the news page is displayed so I have a feeling within the $testimonials variable I need to define the pages and then somehow filter these the tag as a second step, this that correct?

<?php  if ($newspage = page('news')): ?>
    <?php $testimonials = $newspage->children()->listed()->filterBy('tag', 'testimonial')->limit(1); ?>


        <div class="container testimonial bottom-margin">
            <div class="row">
                <div class="">
                    <?php foreach ($testimonials as $testimonial): ?>
                        <h1>Testimonial Title</h1>
                    <?php endforeach ?>
        </div><!-- .container -->
<?php endif ?>

Thank you,

Ignore me, just realised my mistake by not specifying I am filtering by tags - Have updated my original post to correct this issue.

Thank you,