Help URL not working in site.php

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Hey there! I added the following line to config/site.php to specify a bespoke help url but for some reason Concret5 is ignoring it and refers users to the Concrete5 help forum....

Any ideas?

<?php define('MENU_HELP_URL', 'http://mysite.com/page/sub-page/');?>


aarghh.. I need this working by this evening as the site is going live..

Cheers
Cliff