custom form, captcha always succeeds

I have followed the instructions for adding captcha to a custom form.
The captcha field displays fine.
When I validate it, the captcha response is always success, even if it is wrong or blank.
I am fairly good with PHP/HTML, so I am confident I have done it correctly.
The wordpress site is running in a subdomain – could this be a problem?

Otherwise, my form starts:
<form name="myform" action="../process2.php" method="POST">

The validation performed in process2.php is:

session_start();

//Check whether the form has been submitted
if (array_key_exists('check_submit_signupnow2', $_POST)) {

  unset($_SESSION['source']);

  // validation

  // first name is mandatory
  if ((!isset($_POST['process_fname'])) || ($_POST['process_fname'] == '')){
    $_SESSION['error'] = 'Please enter your first name';
    $_SESSION['source'] = 'ERROR';
  }
  // check CAPTCHA code
  if( function_exists( 'cptch_check_custom_form' ) && cptch_check_custom_form() !== true ) {
    $_SESSION['error'] = 'Please enter the correct Captcha code';
    $_SESSION['source'] = 'ERROR';
  }

etc etc

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I know it all fits together well, as if I leave the firstname blank, my form redisplays correctly with the error message.
However, the captcha never fails, even if its wrong or empty..

You can try it yourself here: https://sentinel.vanstonetrading.com/ and just press either of the buttons on the home page

Can you help me?

Cheers,
Bruce