Googlebot can't access your site

I am receiving the following email from Google about several of my websites using this plugin.

http://blarblarblar.co.uk/: Googlebot can’t access your site

Over the last 24 hours, Googlebot encountered 20 errors while attempting to access your robots.txt. To ensure that we didn’t crawl any pages listed in that file, we postponed our crawl. Your site’s overall robots.txt error rate is 71.4%.

You can see more details about these errors in Webmaster Tools.

I have checked the robots.txt file in Google Webmaster Tools and it is reading ok.

Many thanks in advance for assistance.
Kate

http://wordpress.org/extend/plugins/wordpress-seo/