Mapping external users to Windows users using Enterprise SSO API

I'm attempting to use Enterprise SSO to map non-Windows users 
(username/password supplied in the XML body of a BizTalk message) to a 
Windows account to access a web service that uses Windows Integrated 
Authentication.

I'm using the cl***** in BizTalk SSO API 
(Microsoft.BizTalk.SSOClient.Interop).

Initially I tried to issue a ticket but couldn't see an obvious way of 
passing in the external username/password:
            // Get SSO Ticket
            ISSOTicket ticket = (ISSOTicket) new SSOTicket();
            ticket.IssueTicket(SSOFlag.SSO_EXTERNAL_TO_WINDOWS);

I then focussed on trying to log in the external user:
            // Create lookup object for mapping SSO credentials
            ISSOLookup2 lookup = (ISSOLookup2)new SSOLookup();

            // Lookup Windows user details from on external user
            string[] myCredentials = { myPassword };
            long windowsHandle = lookup.LogonExternalUser(myApplication, 
myUsername, SSOFlag.SSO_EXTERNAL_TO_WINDOWS, ref myCredentials);

This correctly identifies the external user, but results in the following 
System.Runtime.InteropServices.COMException:
            Failed to connect to the LSA server.

Am I missing some security setup on the BizTalk Service User account or is

there another way for me to achieve what I am looking for?

Any help greatly appreciated.
Graham