I think I may have sussed it.
I have the Coffee House saved as an IE 'Favourite' and that's how I've always accessed it - i.e. I click on 'Favourites' and open the CH in a new tab.
Yesterday, each time I opened the CH I was routed to either the log-in screen or the main Forum menu page. If I landed on the log-in screen, I had to log-in; if the forum menu opened, I didn't.
This morning I happened to notice that the web addresses for the forum pages have changed. I replaced the old 'Favourites' link with the new one and all has been sweetness and light so far.
I don't know whether this was related to my having to sign in a few times, but it hasn't happened at all today.