Thanks GWS, that did it.
I knew I'd seen a video of it somewhere, just drew a blank- good to know thanks.
The funny thing is - without a crop object I found it impossible to click-select or double click to edit the viewport, Alt-Drag and Drag Marquee to select work and its position can be edited in the OIP.
Also, there seems to be no way to drag it in the Z direction to adjust the elevation (although again OIP Z edits are recognized)
Finally, my crop object seems to calculate as a planar graphic on the new plane but then draws as a screen object, meaning the crop object doesn't 'stick' with the viewport contents and any non-normal view produces anything from unintended cropping to complete loss of the viewport contents.