Google fee load API returning empty rss entries

Question

I have generated the following rss feed which validates correctly with w3: http://www.ppsgroup.co.uk/blog/rss validation here http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fwww.ppsgroup.co.uk%2Fblog%2Frss

However when I try to make an API call https://ajax.googleapis.com/ajax/services/feed/load?v=1.0&num=100&q=http://www.ppsgroup.co.uk/blog/rss200

(I've added a number at the end of the url because it seems google caches the response) I get either an empty entries field or a 400 error.

Any idea what is wrong with the rss feed or how can I debug the api call?

This was working until recently so I'm not sure if something changed on google's side.