A lot of posts I don't want to read, so I check the "Mark all as read" box, but next session the posts all come up as unread again, except for those that I have opened. Is this normal? It would certainly tidy up my page if that button worked for me.

Thanks for pointing out that very useful button, will see what result I get

After using the "Mark all as read" button, I logged off. When I logged back on 10 minutes later, the posts that come up as "unread" (in bold black) have had new replies, all the others appear as read
Hope this helps
It's a good way to keep track of the posts you are following