Problem solved! My page had in the HTML:
<form class="Sharp_Default" id="SimpleContact_Sharp_Default" name="SimpleContact_Sharp_Default" method="post" action="thankyou.php">
(note the action="thankyou.php") which meant on submission the form was bypassing the php email triggers.
HTML changed to:
<form class="Sharp_Default" id="SimpleContact_Sharp_Default" name="SimpleContact_Sharp_Default" method="post" action="">
and the email began arriving.