Safari Yahoo problems

I have Safari Version 12.0.2 on a MacBook Air using High Sierra 10.13.6.

I seem to have got one of those viruses/bugs that redirects my Google searches to Yahoo, be that from Google as a website or from a search term that I type directly into the browser's address bar.

Google remains set as the default browser - so this is not a case of that.

What is more, I have no extensions at all, and so there is nothing that I can do to delete them, since I do not have any.

Beyond that, I have tried the Go To ~/Library/Preferences/com.apple.Safari.plist and removed com.apple.Safari.plist and others like it... in a bid then to quit Safari and to relaunch it.

Except that Safari relaunches on its own, which I think therefore must be part of the bug/virus that does not allow me to remove it.

So... any help would be warmly appreciated. Especially if it does not mean that I have to spend money, since I am kind of penniless right now.

With best wishes

William