It seems like Image sizing constraint is not 'taking'.
After noting that holding the SHIFT key on a corner anchor when changing the placed size of an image did not constrain proportions, I called up Preferences.
In the Tools preference, I found only the 'Move Tool Aspect Constrain' which sounds like it is the one I wanted, but not entirely. I changed the pref for this to 'Constrain by default' - prompting a dialog to restart the App (whereas really, this should be a spontaneous change not requiring an App restart). Having done so, I note that proportions on a placed image are still not constrained when changing the placed size on the page.
