Could not find a key matching one of: "" in the $databases[nhsnwlondon] array in local_databases.php in the directory above Drupal's index.php. This usually happens when you have more than one site URL in the databases array, e.g. $databases[nhsnwlondon][""], $databases[nhsnwlondon][""], etc. If you are seeing this error in Drush, try running the command again with --uri=[URI] where [URI] is a domain name which matches one of the keys in the $databases[nhsnwlondon] array.