Thanks for the replies guys.
I don't get it - none of my plugins have any non-essential GMSTs!
[Game Files]GameFile0=Morrowind.esmGameFile1=Tribunal.esmGameFile2=Bloodmoon.esmGameFile3=Morrowind Patch.esmGameFile4=Morrowind Advanced.esmGameFile5=Wizards' Islands - Scourge of the Frost Bringer.esmGameFile6=Animal Realism.espGameFile7=Morrowind Advanced Patch.espGameFile8=Weapon Compilation.espGameFile9=Wizards' Island Patch.espGameFile10=Enough ''Does Not Have Enough''.espGameFile11=Creatures.espGameFile12=Boots of Infernal Chaos.espGameFile13=Merged Levelled Lists.esp
Wait... I just checked Wizard's Island, and sure enough, there's the sEffectSummonFabricant, and several others (EDIT: 11 in total, which I read is bad...).
Should I just delete this / these from the master file?