Actually much time with the debugger (including using evaluating breakpoint to sneak in an extra timeout on an operation) and I've tracked down my problem to -xmlbeans generating a message with the root element name being of the type of the XSD type (lookupSystemRequest0, but the dispatch (obviously) being based on the name of the SOAP message (lookupSystem).
I will have to think about this, but the problem is clearly in my code...
