Well, unfortunately the problem hasn't gone away.
While I've got rid of the Yahoo issue, other things continue to plague Safari.
So far the following has happened when I've opened a thread in Cyclechat or LFGSS or a grauniad link.
It displays the page in (almost) plain text with no images.
Firefox appears to be unaffected.
Since this morning, I have also installed Big Sur (Oh my! Those colours!)
It has bought the plain text issue with it.
I looked at the scan report from Malware bytes and deletd the six files which had been quarentined.
I've since run Malwarebytes again and the scan isn't coming up with anything.
This is what displays when I click on a thread on LFGSS
I welcome any suggestions for potential solutions.