previous next don’t work when static page set

I have a static page set for my front page and an empty page selected for my posts page as per the guide here http://codex.wordpress.org/Settings_Reading_SubPanel

Now the previous_post and next_post links only display on pages that have been explicitly called they do not show up on the page called by the named posts page (in my case News). So the page at mydomain/News displays the latest post but does not display the Previous link, the same page called as mydomain/postname or mydomain/?p=1234 for example displays the post including the Previous link.

I’m guessing WP would be tryign to identify the next and previous to News rather than the post called in News but how do I get it to behave correctly?

BTW the code in index.php seems to be OK because I do get the vertical bar separator show up that would normally be between the Previous and Next links even though the links themselves don’t show.