Hi Simon,
Unfortunately when a static front page is in use (with Canvas and a few other themes) this problem can crop up. We hope to address it in a forthcoming maintenance release, but in the meantime can you try adding this snippet to your theme’s functions.php file (or wherever else your theme might make available for custom code)?
https://gist.github.com/jazbek/5959855
Note that the opening <?php tag is generally unnecessary and shouldn’t be copied and pasted across.
Does that help at all?