For anyone running into a similar problem, it came down to a plugin conflict; in my case Capability Manager Enhanced and Access which is part of the Toolset plugin. Unfortunately no error_log of /var/log/httpd log files were generated as it was an internal permissions conflict within WordPress.
If you find yourself in this situation, simply FTP / SSH into the server and rename / delete the plugin you don’t want from the /wp-content/plugins/ directory and reload your Admin -> Plugins page.
I hope this helps someone!