Took me a while to figure this out today, so I’m posting in the hope that this helps someone out.
Below is the config for Roundcube Mail (which is wicked by the way) within main.inc.php. One of the important things here was that without an OU, the search returned no results if left with the default LDAP port of 389. To search the whole organisation (without an OU) you need to change to the Global Catalogue port for Active Directory of 3268.
Hope this helps someone!
$rcmail_config['ldap_public']['MyLDAPAddressBook'] = array(
'name' => 'MyLDAPAddressBook',
'hosts' => array('192.168.1.1'),
'port' => 3268, // IMPORTANT!! - Change this from default to Global Catalogue Port
'user_specific' => true,
'bind_dn' => 'firstname.lastname@example.org',
'bind_pass' => 'myLDAPPassword',
'writable'=> false, // Indicates if we can write to the LDAP directory or not.
'search_fields' => array('mail', 'cn'),
'name_field' => 'cn', // this field represents the contact's name
'firstname_field' => 'givenName', // this field represents the contact's first name
'surname_field' => 'sn', // this field represents the contact's last name
'email_field' => 'mail', // this field represents the contact's e-mail
'scope' => 'sub', // search mode: sub|base|list
'filter' => '(&(mail=*)(!(msExchHideFromAddressLists=TRUE)))' , // all mail, except the exchange hidden
'sort' => 'cn', // The field to sort the listing by.
'fuzzy_search' => TRUE); // server allows wildcard search