hmm, shouldn't be anything going to the screen, you could try adding exit();
eg
<?php if ($_SESSION['UserID'] = 6) {
header("Location: http://www.domain.co.uk/product_contents_results.php"); /* Redirect browser */
exit();
} else {
header("Location: http://www.domain.co.uk/news_results.php"); /* Redirect browser */
exit();
} ?>