New tab in browser won't allow set to Google

I've recently migrated work from a 2013 iMac to a 2020 as had lots of issues and thought maybe a failing hard drive. Within 3 weeks one of the very same issue has reared its ugly head so now wondering if I've migrated the same problems over to my new Mac... in Google Chrome, all of a sudden a new tab won't allow the default search engine to be Google and there's no way of changing it in the settings - Google is greyed out. When I search in the URL field the results are from https://uk.search.yahoo.com/ I opened Safari to try that and the same thing happens so not a Chrome only issue. I restarted the other day when it first happened and it resolved itself and was working OK for a day or so, but soon reverted and now a restart hasn't cured it (no that I want to be restarting each time it happens anyway). Can anyone help or advise please? Thanks