>The service is instantiated in my form ( once in the c'tor ) it's the biz class which gets instantiated by my service - the session is terminated in the form closing event.

So I take it that you are creating a ServiceHost instance in Winforms? Then, if using sessions, it will create a new service instance each time a client opens a channel. Does that explain what you are seeing ? If you don't want that to happen then you need InstanceContextMode.Single.

If you do want PerSession behaviour it might be a good idea to also set the SessionMode to 'Required'. In that case if the binding doesn't support sessions then the service will throw an exception. See 'Sessions Interact with InstanceContext settings' here: http://msdn.microsoft.com/en-us/library/ms731193.aspx for the behaviours of different combinations.

HTH finally :-}