Thanks to Ops who has managed to find the UK Film Council report, called "Scoping Study into the Lack of Women Screenwriters in the UK", mentioned in my previous post.
Download the 124 page pdf file directly here.
Scott the Reader has posted something on this issue as well on his blog.
Also see Wall Street Journal article and Guardian article.
Interesting fact: Female screenwriters wrote half of all films copyrighted between 1911 and 1925.