W3C Valid RSS Feed Showing Error in Widget – A feed could not be found

Hi,
My rss feeds have recently started showing an error that I can’t seem to fix. The error indicates that the feed is not valid, yet if I check it in W3C RSS Validator it all checks out OK with no errors or warnings, you can see it here.

Here is an example feed URL from my site: http://www.workformums.co.uk/job-type/home-based/feed/ and the error I get using wordpress standard RSS widget is:
RSS Error: A feed could not be found at http://workformums.co.uk/job-type/home-based/feed/. A feed with an invalid mime type may fall victim to this error, or SimplePie was unable to auto-discover it.. Use force_feed() if you are certain this URL is a real feed. – See more at: http://www.workformums.co.uk/#sthash.Be2ZHPM4.dpuf

Website URL: http://www.workformums.co.uk
The RSS feeds are in the footer widget area.

Any help appreciated!!
Colin