I struggle to visualise the contents of a SOAP message by looking at WSDL. This is compounded by the somewhat indirect way changing settings in WCF service configuration (either in configuration files or code attributes) affects the WSDL and ultimately the SOAP message. It is much easier to actually have a peek at the contents of the SOAP message.
The solution to my problem was provided by Kory Becker in his article Displaying SOAP XML Messages in a Simple WCF Web Service.
For convenience I’ve created a little Visual Studio project (C#) with the code already implemented here. Note that to see the output in Visual Studio for services hosted in IIS the trick is to debug the service by attaching the debugger to the ASP.Net worker process.