Thank you for the advice. I have just tried in safe mode and exactly the same thing happened - I opened Safari and used Google to search the string: "Batley Grammar: Parents pull children". When a Sunday Times piece with this headline came up in the search results I did 'command-click' on that result. As soon as I did that a new tab came up with 'Favourites' showing on the tab where normally I would expect to see a title pertaining to the item I had just selected. At this stage my search results were still displaying in the first tab but when I clicked on the new tab to activate it, it displayed my Google 'favourites' as icons as if I'd just opened a new window or tab in Safari but when I re-activated the first tab, the list of search results had been replaced by the Sunday Times page I had selected.
This always happens with the Sunday Times but not with most other web pages. With Wikipedia, it happens maybe 25% of the time. For example, after I restarted my computer just now to return from safe mode back to normal running mode, I opened a Safari window, typed in the string "Helen Hunt" and command-clicked her Wikipedia entry which was the first item in the search results. When I did this it opened up a new tab entitled 'favourites' and when I clicked on that, then back on the search results, they had been replaced by Wikipedia exactly as described above for the Sunday Times. However, as I was typing this, I thought I would try Helen Hunt again in a new search and did exactly the same as I had 5 minutes earlier but that time, Safari worked normally with the Wikipedia page coming up in the new tab and my search results remaining in the first one.
Though it only happens some of the time with Wikipedia, with the Sunday Times, this happens almost all the time whatever the news story and I seem to remember the Times is the same in this respect. At the moment I have selected 'block all cookies' in Safari which doesn't seem to work as it should either as I still get lots of cookies that I then delete each session. The problem with tabs though happens regardless of whether cookies are blocked or not blocked.
Thanks again for your help.