Thank you for your reply Z. I tried uninstalling and re-installing Java and that made no difference. My normal browser is Chrome.
Fingers crossed, I might have cracked it.......
After lots of Googling, I stumbled across a site where someone complained of the same problem. They were advised to locate Yahoo Mail UK manually via Google, rather than using the bookmark and to see if that worked. For them it did work.
I tried the same thing and it has worked for me! So, I have deleted the old bookmark and bookmarked it afresh.