GRRRRR, I had almost completed the test and selected the option to "zoom in" and it restarted the test!!! I'll try agin later after I'm not angry anymore. I don't believe that I have short term ediectic memory anyway.
I got 40% - and an error connecting to the server so I couldn't see which ones I got right. I suspect the first four, since I think I was using rapid memorisation techniques rather than any intuitive ability to just "see a difference".
10% on the first try, 70% on the 2nd. looking for changes to the biggest continuous block of white squares (& mostly ignoring the rest) seemed to work pretty well on the 2nd try. I don't know if that's cheating or what but those 10x10 squares were too much to take in in such a short time.