Sorry not to answer earlier, but I've just had to spend an hour with the World's Smallest Allen Key™ disassembling, cleaning, drying and reassembling my brand-new BT keyboard, the unwilling recipient of half a cup of hot coffee this morning. Now it's all back together, I can report that the problem, as I suspected, was QT 7.1.3. With Pacifist and some difficulty I have uninstalled 7.1.3 and reinstalled 7.0.4. Everything is now hunky-dory. QT 7.1.3 obviously has some unresolved issues.
I have absolutely no idea, except that it seems to worm its way into just about every media format at some point. I picked up on it from reading Apple Discussions. All I know is that as soon as I had downgraded it, PS 7 could open all the files again.