OK - Not sure about using mktime for it, but this works.
<?php
$today=date("w", time()); //allocates day of week number
if ($today==5) // is it a Saturday?
{
$dayofweek=date("l j M Y", strtotime("last Friday")); // if so show last fridays date
}
$dayofweek=date("l j M Y", strtotime("Yesterday")); // else show yesterdays date
echo $dayofweek;
?>
Tada !! (Well one way of looking at it anyway !)