Greg Wooledge on 1st September very kindly gave me a detailed reply to my cry for help.
I tried to follow his reply but I have lost all my memory following a stroke.
So it took me some time to do so as I had to relearn the basic file commands. I got as far as the following:-
" - bash: home/john: Is a directory
john@john-PC:/$ ls -ld home/john /home/john/.ICEauthority
dr-xr-xr-x 26 john john 4096 Aug 31 11:46 home/john
-rw------- 1 john john 9016 Aug 31 11:21 home/john/.ICEauthority "
This takes me no further forward.
What puzzles me is that the login worked at least 30 times without the error message and I cannot understand why it stopped.
This takes me no further.