a couple things to try to change:
define('HTTP_SERVER', 'http://www.railworks.marleyman.co.uk');
define('HTTPS_SERVER', 'https://www.railworks.marleyman.co.uk');
and
define('DIR_WS_CATALOG', '/store/');
define('DIR_WS_HTTPS_CATALOG', '/store/');

plan b
in the admin
configuration->sessions->Add Period... change that setting

either one of theses work?