I just added this bit of code to the header (as a test, it should go in the correct php file probably):
<?php
header("Cache-Control: no-cache, must-revalidate"); // HTTP/1.1
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT"); // Date in the past
?>
and it appears to have sorted the issue, so was it indeed some form of server side caching? What do you think?