Graham,
Go into the
Facebook Developers area
and click "Edit Settings". Under the "Page Tab Settings" section, change the "Page Tab Width:" to "Normal".
That should fix you up, but if you have any questions, just let us know. Also, please note that your site doesn't look to be using SSL. Most users will not be able to see your Page Tab until you install an SSL certificate. For more information on how to get one, see our
SSL certificates for Facebook
.
Good luck,
Alex