Hi Neil,
While I see the problem when I visit your site I seem unable to replicate the same thing locally.
I realize you indicated as much already when you opened the topic, but can you confirm you did indeed follow the troubleshooting steps outlined here (principally, deactivating all other plugins and switching to a default theme)?
If not, it would be appreciated if you could do so. I see an error, but it’s not one that happens for me under default conditions.
Beyond that, if you are comfortable doing so and could add:
define( 'SCRIPT_DEBUG', true );
To your site’s wp-config.php file, that would be useful and might help us track down the source of the error.
Thanks!