We can see that the sitemap source code is generated correctly by viewing the page source of the following non-pretty permalink URL: http://barbernbar.co.uk/?sitemap=1
This means something outside of our plugin is causing the redirect. Sometimes strange behavior may be caused by a plugin or theme conflict. The fastest way to rule out conflicts is to deactivate all non-Yoast plugins and switch to a standard theme like Twenty Seventeen. If this resolves the issue, you have a conflict and should activate one plugin at a time until you locate the conflict. Finally, after all plugins are active, switch to your theme. This will narrow down the conflict to a specific plugin or theme.
If you are unfamiliar with checking for conflicts, we wrote a small guide that will walk you through the steps. You can find it here: How To Check For Plugin Conflicts
Thanks for your help. I disabled all plugins and re-activated them one by one and now it seems to work! 🙂
