I noticed suddenly appeared in my Google Webmaster tools 404 errors for pages not found... they refer to urls which are a 'mirror' image of my blog but created in a folder called "blogging" which didn't exist until about 3 days ago.... but doesn't contain any of the blog files.
Has anyone any idea what could have generated this folder automatically?
I've never heard of this issue with WordPress. Is is possible that you did something by accident? Have you made any edits to your htaccess file or added any new plugins or other adjustments to the WordPRess or Hosting setup?
Nothing has been "created" on your site if the URLs return 404. What has happened is that somewhere on the web there are now some links pointing to incorrect URLs. Wait for the WMT data to further update to include the backlink data. The links might be on some other site.
Ah, you posted again. Looks like someone made an error then corrected it by removing the duff stuff. But unfortunately not before Google had been in there. Set up a RewriteRule to mass redirect the incorrect URLs to the matching correct URL.
This redirect will appear very near the start of your htaccess file. RewriteRule ^blogging/(.*) http://www.example.com/$1 [R=301,L]