Hi Again,
Followed that through, adding $request_type = ($_SERVER['SERVER_PORT'] == '443') ? 'SSL' : 'NONSSL'; to root and application_top but still the callback failed. ( my _SERVER["SERVER_PORT"] value is 443 so I'm assuming this line is correct).
Reconfigured the WorldPay callback url to http: as opposed to https: and the callback works.
Regards,
Graham.