Just to punish me for complaining, the shot I took this morning (left in a drawer to develop while I ran off to work) is clean as a whistle! Well developed to the edge, maybe a few bits of weirdness right at the very border but nothing significant. I apparently even managed to hold the camera steady for an exposure that was way longer than I planned for. So maybe the secret is to complain a lot and wait for the Imp of the Perverse to take revenge.