Solution:
I found I had to explicitly include the ‘wp-admin/includes/screen.php’ script
<?php
if(!class_exists('WP_List_Table')){
require_once( ABSPATH . 'wp-admin/includes/screen.php' );
require_once( ABSPATH . 'wp-admin/includes/class-wp-list-table.php' );
}
/**
* Handle the admin tasks for the BHAA companies
* @author assure
*
*/
class MyTable extends WP_List_Table