I see the log now but I had to double-click .scr to generate it as per my previous post.
I have just checked it now and it is showing a run from last night even though this morning I have logged on my PC and left it for a while. When I returned the secondary screensaver was showing so I assume JKDefrag had run but this is not showing in the log.
EDIT: I just watched it run and go to the secondary screensaver, SunClock6. The log is still showing the run from last night. I can get the log to update by double-clicking the .scr file but leaving the screensaver to run naturally does not update the log.
My son's PC is showing the same behaviour. His PC has just finished the screensaver defrag and there is no log. I created one by running the .scr file.
EDIT2: Elementary, my dear Jeroen. It's working. One has to look at the log-file in the system32 folder. Too used to using the command-line log