Thanks to all who gave constructive advice.
The problem was not cured by adblocker.
I made several attempts to use various malware/virus removers with no success.
I had previously removed what might be Chrome extensions causing the problem. That had no effect, the extension was not visible.
I did go to Chrome Advanced/ Reset and did the reset. That seems to have done the job.
I have no clue however where the villian was hiding
Russia....................