As this is my first VI using refnum's I am confused about how this error can occur as the refnum was created by right clicking on the invoke node and selecting Cayman.ICayman from the registered options. The only reason I created the control was due to the invoke node requiring a wire to reference in order to run the VI.
All you've done is pass in an empty reference. The type of the reference may be of the type "Cayman", but its value is null. Hence the error. You must actually create a valid reference. With an ActiveX component this is done using the Automation Open function. Thus, you have to insert an Automation Open function between the control you created and the Invoke node.