Philsy on "Unable to add blog posts"

Hi

I'm trying to add a blog page to an existing website. I've added the page and set it up as the landing page for blog posts. However, when I then create a post I get the following:

Fatal error: Call to undefined function martinsewell_posted_on() in /var/www/vhosts/msbc.uk.com/httpdocs/site/wp-content/themes/martinsewell/loop-single.php on line 29

I've looked on functions.php and found the following around line 29:

*
* add_action( 'after_setup_theme', 'my_child_theme_setup' );
* function my_child_theme_setup() {
* // We are providing our own filter for excerpt_length (or using the unfiltered value)
* remove_filter( 'excerpt_length', 'twentyten_excerpt_length' );
* ...
* }
*

Can someone suggest what is wrong, please?

Thanks

Phil