We have an RSS feed (or we should have) via wordpress but it is just giving using a 404 Page Not Found on load - http://www.warrenaccess.co.uk/feed/rss2
We have tried changing back to default theme and deactivating all plugins to see if we could find an issue but it still didn't work.
Does anyone have any idea what could be causing this?
My hunch is that you don't actually have any blog posts, but you are using WP as a pseudo-CMS. From having looked at the feed and network conditions (CDN / cloud hosting can sometimes ignore feed content), things are working as they should.
Your 404 does provide some useful information about the feed:
<lastBuildDate>Fri, 11 May 2012 10:14:42 +0000</lastBuildDate>
Which is awhile ago, relatively speaking. I'm guessing that your site content is all based on pages, which don't and shouldn't actually show up in RSS feeds.
To fix the problem, you'd have to make some blog posts or try this plugin or its variant:
http://wordpress.org/extend/plugins/rss-includes-pages/
Edit: turns out WP syndication setting was set to show -1 posts. Fixed by setting it > 0.