A good place to start would be
http://www.mediawatchuk.org/mainsite.htm
whihc details how and why the National Viewers and Listeners Association which was co founded by Mary Whitehouse CBE was setup. Basically it was an orgainisation to pressure the TV and Radio networks (not may around then!!) to keep their content, good clean and Christian without undue sex and violence. I can't directly identify that this was the reason for the 9pm watershed although I would imagine that the government proportioned the viewing day into various periods (with suggestions of content) and made it a condition of the broadcasters' licences to honour it. Cheers