The bread crumbs indicated I was in Community>Discussions and I wanted to create a new post, and couldn't see an option to do it (which was strange because I had just made a post!). The only big fat button I could see was "Join A Discussion". I didn't really want to join a discussion but I thought I would click it to see what it did. And here I am, now creating a new post. If this wasn't an experimental system I probably wouldn 't have clicked the button!
Having posted this question the bread crumb is now "Community>Discussions>I Just Got Totally Lost". Now I have a big fat button to "Post A Question". This is what I was looking for earlier. But what it does is to allow me to "join the discussion" by posting a reponse. I was on a screen where logically the only thing I would do it to reply to the open discussion - I would expect to either reply here or go up a level to post a question.