I see your sitemap at that address so I think it's getting rendered fine.
However at the bottom of the sitemap source code I see the following code:
<!-- Performance optimized by W3 Total Cache. Learn more: http://www.w3-edge.com/wordpress-plugins/
Minified using disk: basic
Page Caching using disk: enhanced
Database Caching using disk: basic
Object Caching 246/298 objects using disk: basic
Served from: www.freakfish.co.uk @ 2013-01-31 22:16:01 -->
That code might be the issue. You should stop that from appearing on that page because that's probably the code that's causing the problem and it's not even successfully caching the page (from what I can tell).