Thank you for your reply. It seems after rebooting my computer this morning and retrying it, the function is now behaving as I anticipated it should and I do get two measuring dots. Not sure what happened there.
However, I do have to load something onto the build plate in order to get the left menu bar (showing the plug-ins) to appear, and then the measuring tool is active without selecting the model. Loading a model brings up the measurement dialog and it works fine. Closing down that project leaves me with an empty build plate, and the two dots are still there but useless with no data dialog open. Of course, I don't care at that point because it only needs to work when there is something to be measured.