I\'ve create a regular old ASMX web service in ASP.NET and added SoapDocumentMethod(OneWay = true)] to the function call as I\'ve read this is supposed to make the call Asyn
Are you returning a value? The documentation for OneWay states that "fire and forget" web methods cannot return a value or have out parameters.
http://msdn.microsoft.com/en-us/library/system.web.services.protocols.soapdocumentmethodattribute.oneway(VS.80).aspx