Okay, so I gave into my impatience and tried opening it up. I now have different ... issues. One of the first things I noticed was there was a short, tiny screw floating around.
Anyway, I put that to one side and tried had a deeper look. Nothing particularly worrisome was to be found. After poking around I put it back together (have you tried turning it off and on again?) and put it back on the K-30. Hey! The aperture works properly now; great (though the VF still goes dark when I stop down)! Oh, hang on, I left the end on the table - that funny bit that the aperture lever sits against. I put that on and now the aperture won't close past F4.
So at the moment I have an incomplete lens which opens and closes fine, but is missing the end ring thingy (and a screw from somewhere?) and darkens the aperture when it stops down. It's fine for live view or up to F5.6 I guess, but doesn't feel right.