A recent hosting customer is using Linnworks to post their catalogue to ebay, amazon and Magento. Despite following the guide lines here they were still unable to connect to their site. Eventually the Linnworks support were able to offer the following.

in lib/Zend/XmlRpc/Server.php

comment out:

if (!$matched)
{
#require_once 'Zend/XmlRpc/Server/Exception.php';
throw new Zend_XmlRpc_Server_Exception('Calling parameters do not match signature', 623);
}

hope this helps someone/