Having spoken to cyfm it seems the PageStream Function key mapping can be modified using the Script Palette so that a modifying "Special" key (such as Control, or several other key combination(s)) can be added so they do not clash-conflict with the iMac F1/F2 key(s) to set the Brightness level.

Hopefully this is an interim solution, as I would prefer to have the original PageStream single 'F' key as usual, rather than a 2 key pressed option, but it does at least free the clash-conflict for the time-being that occurred when I installed MorphOS v3.18.