Thanks, bagginsbilol, for your suggestion. Why didn’t I think of this.... doh! 

Quote - 2.2 for PC, 1.8 if you're on a MAC

1.8 is the default gamma setting for Macs when you take them out of the box, which is a figure set for print work. However it seems to be fairly common practice these days to set Mac gamma to 2.2, especially if you are working primarily on web sites and you need to be sure that your images will work properly on Windows machines. Working to a gamma of 1.8 can make your colours seem very dark on a Windows machine.

Should the gamma settings in Poerr Pro’s render settings match the gamma setting for your monitor profile rather than the "out of the box" figure? I’m giving this a go, BTW, and so far I think it’s making a positive difference.