Services 3.x REST server create update without storing NID on remote site

Im just at the begining of trying to understand services and rest, I have a simple php script that I can use to create resources from my remote site, this works fine as does updating resources when I have the NID to add into the script.

What I cant seem to get my head around is how I link create and updates from a remote site when the remote doesnt have or store the NID, I assumed that perhaps using CCk to store a Unique ID from the remote may be the answer but Im unsure how I would go about retrieving a NID from a REST call with a Unique CCK field.

The following code will create the entry in my test site no problems, if I append the http://mysite/course_rest_service/node/ with a node ID and change the curl op to CURLOPT_CUSTOMREQUEST 'PUT' this will update the node without any problems.

Im just not sure how to use services to find out if there is a field_unique_id(CCK Field with this value in that content type) get the NID for it to run the update operation.

* Server REST - user.login
*/

// REST Server URL
$request_url = 'http://mysite/course_rest_service/user/login';

// User data
$user_data = array(
'username' => 'name',
'password' => 'pass',
);
$user_data = http_build_query($user_data);

// cURL
$curl = curl_init($request_url);
curl_setopt($curl, CURLOPT_HTTPHEADER, array('Accept: application/json')); // Accept JSON response
curl_setopt($curl, CURLOPT_POST, 1); // Do a regular HTTP POST
curl_setopt($curl, CURLOPT_POSTFIELDS, $user_data); // Set POST data
curl_setopt($curl, CURLOPT_HEADER, FALSE); // Ask to not return Header
curl_setopt($curl, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($curl, CURLOPT_FAILONERROR, TRUE);

$response = curl_exec($curl);
$http_code = curl_getinfo($curl, CURLINFO_HTTP_CODE);

// Check if login was successful
if ($http_code == 200) {
// Convert json response as array
$logged_user = json_decode($response);
}
else {
// Get error msg
$http_message = curl_error($curl);
die($http_message);
}

// REST Server URL

$request_url = 'http://mysite/course_rest_service/node/';

// Node data
$node_data = array(
'field_unique_id[]' => array ('value' => '718000021955232'),
'title' => 'this should update 5537',
'type' => 'coursefeed',
'body' => '

losts my body

',
'taxonomy' => array(1=>array(19)),
'field_about_this_course[]' => array ('value' => '

information about a course

$node_data = http_build_query($node_data);
echo $node_data;

// Define cookie session
$cookie_session = $logged_user->session_name . '=' . $logged_user->sessid;

// cURL
$curl = curl_init($request_url);
curl_setopt($curl, CURLOPT_HTTPHEADER, array('Accept: application/json')); // Accept JSON response
curl_setopt($curl, CURLOPT_POST, 1); // Do a regular HTTP POST
curl_setopt($curl, CURLOPT_POSTFIELDS, $node_data); // Set POST data
curl_setopt($curl, CURLOPT_HEADER, FALSE); // Ask to not return Header
curl_setopt($curl, CURLOPT_COOKIE, "$cookie_session"); // use the previously saved session
curl_setopt($curl, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($curl, CURLOPT_FAILONERROR, TRUE);

$response = curl_exec($curl);
$http_code = curl_getinfo($curl, CURLINFO_HTTP_CODE);

// Check if login was successful
if ($http_code == 200) {
// Convert json response as array
$node = json_decode($response);
}
else {
// Get error msg
$http_message = curl_error($curl);
die($http_message);
}

print_r($node);

Help, direction or better understanding of how this could be implemented easier would be greatly appreciated