Hi Angus,
I took a look at your site, and I wasn’t able to create a new page in the wiki either. I noticed you had an add-on installed that auto-generates an .htaccess file for you. Since we recommend instead using the instructions listed here for removing index.php from your URLs, I tried that instead, renaming your current .htaccess file to .htaccess-old. After I did that, everything worked just fine!
I left the working .htaccess file in place, but you’ll want to deactivate that add-on as well so that it doesn’t overwrite the working .htaccess file.
That should have you all fixed up! Is there anything else I can help with?