Contact 7 form wpcf7_before_send_mail formdata omits hiddenen fields

Hi. I have a contact 7 form with definition including:

[hidden text ID default:get]
<label> Your Name (required)
[text* your-name] </label>

The hidden ID field is populated from the ID field passed in the URL to the WordPress page (I am using a custom link)
Within the PHP code handling the form submission using wpcf7_before_send_mail I have the normal code:

function contactform7_before_send_mail( $wpcf7 ) {
$form_to_DB = WPCF7_Submission::get_instance();
$form_id=$wpcf7->id();
if ( $form_to_DB ) {
$formData = $form_to_DB->get_posted_data();
}

This works fine and I can get the form field data out of $formData. However I am trying to get the data in the hidden field using $formData[‘ID’]
This works fine if the ID field is NOT hidden but when hidden returns blank.

I realise this is a little obscure but I have tried for a day to find a way round this.

Thanks Paul