How to only have it appear on the single post page

Ok – you might want this if you also want the same as me – change wp-simple-related-posts/modules/base.php to as per below – the part I added is the check for us being on a single post

if(is_single()) {
			return $content.$this->get_reloated_posts();
				}
			else
				{
				return $content;
				}

Full file

<?php
class Simple_Related_Posts_Base {
	protected $option = '';

	public function __construct() {
		$this->option = get_option('sirp_options');
		if ( $this->option['post_content'] == 1 )
			add_filter( 'the_content', array($this, 'the_content') );
	}

	public function the_content( $content ) {
		if(is_single()) {
			return $content.$this->get_reloated_posts();
				}
			else
				{
				return $content;
				}
	}

	public function get_reloated_posts( $num = '' ) {

		$posts = $this->get_data( $num );
		if ( !$posts )
			return false;

		$html = '<h2 class="simple-related-posts-title">' . apply_filters( 'sirp_title' , __( 'Related Posts', SIRP_DOMAIN ) ) . '</h2><ul class="simple-related-posts">';
		foreach( $posts as $post ) {
			$html .= '<li><a href="' . get_permalink( $post['ID'] ) . '" >';
			$html .= apply_filters( 'sirp_before_post_title', '', $post['ID'] );
			$html .= apply_filters( 'sirp_post_title', '<p class="title">' . get_the_title($post['ID']) . '</p>', $post['ID'] );
			$html .= apply_filters( 'sirp_after_post_title', '', $post['ID'] );
			$html .= '</a></li>';
		}
		$html .= '</ul>';

		return $html;

	}
	public function get_data( $num = '' ) {
		$related_posts = $this->get_data_post_meta($num, get_the_ID());
		if ( !$related_posts )
			$related_posts = $this->get_data_original($num, get_the_ID());

		return $related_posts;
	}

	public function get_data_post_meta( $num = '', $posts_id = null ) {
		global $post;

		if ( !isset( $post_id ) )
    		$post_id = $post->ID;

    	if ( empty( $post_id ) )
			return false;

		$posts = get_post_meta( $post_id, 'simple_related_posts', true );

		if ( !$posts )
			return false;

		$posts_ids = array();
		foreach ( $posts as $id ) {
			$posts_ids[]['ID'] = $id;
		}

		$current_num = count($posts);
		if ( $num == '' ) {
			$option = get_option('sirp_options');
			$display_num = intval($this->option['display_num']);
		} else {
			$display_num = intval($num);
		}

		if ( $current_num > $display_num ) {
			$num = $current_num - $display_num;
			for ($i = 0; $i < $num; $i++) {
				array_pop($posts_ids);
			}
			return $posts_ids;
		} elseif ( $current_num < $display_num ) {
			$num = $display_num - $current_num;
			$posts = $this->get_data_original( $num );
			if ( !$posts ) {
				return $posts_ids;
			} else {
				return array_merge($posts_ids, $posts);
			}
		} else {
			return $posts_ids;
		}
	}

	public function get_data_original( $num = '' ) {
	}
}

Thanks You.
Please pull request below
https://github.com/horike37/Simple-Related-Posts