I identified the problem as being the Yoast SEO 6.2 plugin – when I deactivated this, everything worked.
include/mail.php seems to be doing the job! I have it creating a test account now via the rest API call.
Next question is how to obtain if the value set by a selection box ie. validate if user selected Yes/No? I have the form submitting the checkbox value, i can see the value yes/no in the email sent to the ower of the site but I need to take it out of the values being sent to the method
public static function send( $template, $name = '' )