thanks for your response zenofmud.
So I would use a Change node (for example) to add the boolean property to the message, and then use a switch node to test that property? It seems like an extra step than should be needed, but if it works, I'm happy. However, I still have the same issue in the change node in referring to the input_boolean value. JSON, Jsonata etc are all a bit of an unknown to me. However, if it I could work-out how to read the value of the input_boolean in a Change node, I could probably use the same method to refer to the input_boolean in the switch node? - and miss the extra step of the Change node ?