Managed to figure it out.
The pdf output code has this to get the normal field values:
$form_sql = "SELECT * FROM `".$wpdb->prefix."wpsc_submited_form_data` LEFT JOIN `".$wpdb->prefix."wpsc_checkout_forms` ON ".$wpdb->prefix."wpsc_submited_form_data.form_id = ".$wpdb->prefix."wpsc_checkout_forms.id WHERE `log_id` = '".(int)$purchase_id."' AND `active` = '1'";
$input_data = $wpdb->get_results($form_sql,ARRAY_A);
The there is a switch statement based ont he unique_name but as this is a custom field it does not have a unique name. I added a loop at the top top loop through and look for the form_field id instead which i got from the database (21 i8n this case) and then I was able to use an if statement to check for the id and output it.