Thanks Jason - that worked.
The notes say to enter "UserID" in the dialogue box text field for Session Variable ... so that's what most inexperienced folk (like myself) and probably a good number of more experienced developers are going to do!
Perhaps the notes should be changed at some point to say "Enter the Session Variable you used to create your logon page."
And / Or perhaps the dialogue box could be altered in a future version so that the Session Variable field is changed from a text box to a drop-down selection box displaying all of the session variables available ?
Thanks again for your help!