Get error trying to Update File after modifying header.php

I modified the nav menu in header.php as below(added external link)

<ul id="navmenu-h">
<li><a href="<?php echo get_settings('home'); ?>">Home</a></li>
<?php wp_list_pages('sort_column=menu_order&depth=2&title_li='); ?>
<li><a href="https://twitter.com/0co2caravan"></a></li>
<li class="alignright"><a href="<?php bloginfo('rss2_url'); ?>">Subscribe via RSS</a></li>
</ul>

When I go to save I get

Warning: fopen(C:\Inetpub\vhosts\zerocarboncaravan.net\httpdocs\wordpress/wp-content/themes/monochrome_gallery/header.php) [function.fopen]: failed to open stream: Permission denied in C:\Inetpub\vhosts\zerocarboncaravan.net\httpdocs\wordpress\wp-admin\theme-editor.php on line 55

Warning: Cannot modify header information - headers already sent by (output started at C:\Inetpub\vhosts\zerocarboncaravan.net\httpdocs\wordpress\wp-admin\theme-editor.php:55) in C:\Inetpub\vhosts\zerocarboncaravan.net\httpdocs\wordpress\wp-admin\theme-editor.php on line 70

Has anybody got any idea what is wrong?