Fatal Error when attempting to upgrade to 3.5 from 3.4.2

I’m getting the following error when trying to upgrade from 3.4.2 via ftp.

Update WordPress

Downloading update from http://wordpress.org/wordpress-3.5-new-bundled.zip…

Unpacking the update…

Verifying the unpacked files…

Warning: require_once(/var/www/wordpress/wp-content/upgrade/version-current.php) [function.require-once]: failed to open stream: Permission denied in /var/www/wordpress/wp-admin/includes/update-core.php on line 640

Fatal error: require_once() [function.require]: Failed opening required ‘/var/www/wordpress/wp-content/upgrade/version-current.php’ (include_path=’.:/usr/share/php:/usr/share/pear’) in /var/www/wordpress/wp-admin/includes/update-core.php on line 640

This is not the first time i’ve tried to upgrade – this blog has been upgraded many, many times.