Thanks for ideas.
I have now used Disk Utility to repair disk permissions.
Then tried to access system log, but was denied permission to access.
Now I have repeated using Terminal for sudo periodic runs, but this time daily, weekly then monthly as 3 separate instructions.
Seems to work OK without error message.
So I am keeping fingers crossed that all is well now.
I did stumble into Console Log, but it contained hundreds of error messages, so was not much help.
I could send it to anyone interested.