I alway return to the homepage (the one where you first logging in) and then go to "Your account" which provides drop down menu and hit "Logout". The "Your account" box reverts to a "Login" box. Does this sort the problem?
Thank you, yes it does, sort of. I still think logout should happen automatically when I exit from the browser, or a few minutes later, not several hours as it seems to. And "Your account" isn't the most obvious place to look for Logout.