Thanks for getting back to me :)
Yes, it's working now and feel a bit embarrassed.
in my callback I initially had this code at the top of my php
// Authentication headers for when required
if ($_GET['hub_challenge']) {
echo $_GET['hub_challenge'];
error_log("\n" . '[' . date("F j, Y, G:i e") . '] : ' . "Hub Challenge - Exiting", 3, "logging.txt");
exit;
}
However having that error log line stops the hub challenge from working. (Not sure why it would stop the challenge/verification but whatever :) )
Removing that line to make it this
if ($_GET['hub_challenge']) {
echo $_GET['hub_challenge'];
exit;
}
it now works a treat :)