I made the suggested changes to the sendemail.php file, so the entire file now contains...
<?php
$name = @trim(stripslashes($_POST['name']));
$from = @trim(stripslashes($_POST['email']));
$subject = @trim(stripslashes($_POST['subject']));
$message = @trim(stripslashes($_POST['message']));
$to = 'mail@URLREMOVED.co.uk';
$headers = "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/plain; charset=iso-8859-1\r\n";
$headers .= "From: {$name} <{$from}>\r\n";
$headers .= "Reply-To: <{$from}>\r\n";
$headers .= "Subject: {$subject}\r\n";
$headers .= "X-Mailer: PHP/".phpversion()."\r\n";
$sent = mail($to, $subject, $message, $headers);
die;
After attempting another test message, nothing was received in my email inbox at all.