They should be back online again soon.
If you are the host that runs this Magento server being tested, please triple check your sitemap.xml and if you definitely have one and it's still not working email us and we will investigate.
Sitemap link: sip300-uk-demo.nexcess.net
View Raw Siege test results
Could not get sitemap.xml at: http://sip300-uk-demo.nexcess.net/sitemap.xml