Cancel this guys.. I deleted all of my screens for the object, and started again. Miraculously it worked as expected the next time round.
DataList bound to root>Overview01, and Query bound to Root>Overview01>QueryByElements.
Days wasted on a twilight zone problem!!...?
The only factor here is I upgraded to the latest 1302 studio in between the first creation of my screens, and ditching them and trying again. I wonder if there was a problem in the very early build of 1302 studio?
Jon