Thanks for your help. Using the demo site, I was able to reproduce my problem.
Please note I don't have TestStudio. I am using Visual Studio and coding the test harness by hand.
In the video you linked to me, I noticed you used Telerik.WebAii.Controls.Html.RadTreeView for the control, however, I am using Telerik.WebAii.Controls.Xaml.RadTreeView. I found the object using:
(in the above, RadTreeView = Telerik.WebAii.Controls.Xaml.RadTreeView)
And it's not possible to cast the found control to the Html version of the control - only the Xaml version.
I'm a bit confused.