How do I debug "Could not find valid value for ct" error message
OK, turns out this was due to erroneous entries in the Drupal settings.php file. The following entries were causing cookies to be missed by Civi:-

 ini_set('session.cookie_lifetime',  2000000);
 ini_set('session.gc_maxlifetime',   200000);


Others may also cause inconsistencies, such as:-

 ini_set('session.cache_expire',     200000);
 ini_set('session.cache_limiter',    'none');
 ini_set('session.cookie_lifetime',  2000000);
 ini_set('session.gc_maxlifetime',   200000);
 ini_set('session.save_handler',     'user');
 ini_set('session.use_cookies',      1);
 ini_set('session.use_only_cookies', 1);
 ini_set('session.use_trans_sid',    0);


Removing the entries from the drupal settings.php file, then closing the browser and re-establishing a new session has resolved the issue.

Essentially these settings within Drupal are conflicting with the PHP.ini settings that CiviCRM is seeing, which can lead to these erroneous types of session errors.