Thanks for the reply. Here it is:
The reason for the discrepancy in size between this and the previous screenshot is that I have a 500GB external hard drive plugged in now. I'd disconnected it earlier, to see if it would make any difference to the analyser results as the external dive really is 80% full, but no joy.
du: cannot access `./home/lesley/.gvfs': Permission denied
du: cannot access `./proc/3225/task/3225/fd/3': No such file or directory
du: cannot access `./proc/3225/task/3225/fdinfo/3': No such file or directory
du: cannot access `./proc/3225/fd/3': No such file or directory
du: cannot access `./proc/3225/fdinfo/3': No such file or directory