Access denied error trying to invoke SOAP service

Hi,

I've defined a service using Services 3 API creating the endpoint and resource as standard. It is using SOAP Server as the server method, and I'm using hook_soap_server_wsdl to override the default WSDL file.

The WSDL is generating fine, and testing in UI it is WSI compliant and generates the requests correctly. However, when I try to invoke the service using SOAP UI I get the following response:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
   <SOAP-ENV:Body>
      <SOAP-ENV:Fault>
         <faultcode>Access denied for user 0 "anonymous"</faultcode>
         <faultstring>401</faultstring>
      </SOAP-ENV:Fault>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

The endpoint is set with the following access requirements:

'access arguments' => array('access content'),

and I have confirmed that the anonymous user has access to both the endpoint resource and the access content permissions.

Has anyone else experienced this and found a resolution?

Thanks in advance, let me know if more information is needed.
Tom