Ok, here it is.
When I update some setting withing sitemap settings and check the sitemap using 'view your sitemap' it shows me sitemap.xml just as it should.
Now, if I refresh the page sitemap is gone and all I get is 404 from now on.
Google is also unable to find the sitemap when trying to upload it using webmastertools.
Any tips? Link in question - http://www.jfsports.ie/sitemap.xml