'Ok so what I find bizarre here from the is that folks here are so willing to discount the fact that one of the major ISPs has said that this solution - blocking hardcore porn at source - is indeed completely feasible.'
From TalkTalk's Parental Controls 'solution' FAQ:
'2) I have different-aged children in my home; can I have different settings for different ages?
'Due to the way the service works, it provides a single group of settings across any device connected to your home broadband at the time. However, you can change the settings whenever you choose, and customise the settings as you see fit for whatever you believe provides the best balance of protection for your whole family.'
So 'no' then - same settings for 5 y/o, 14 y/o, mum who likes to MN and dad who secretly uses porn. 
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'8) Can I add sites that I personally feel are inappropriate to the Block list for my household/child?
'Yes, you can add up to nine web sites to your own custom list.'
A whole nine websites? I could probably think of more than nine off the top of my head if I tried 
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'10) In contrast, can I add specific sites I feel are appropriate, as exceptions?
'Yes, you can add up to nine custom sites.'
yay, nine ...
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'19) I've blocked a particular Parental Controls category, although I want to visit a site in the category. I've added this into the list of specific sites to allow, but I still can't view the website.
'Some websites redirect you from their main web address. For example, www.twitter.com redirects you to twitter.com. On the other hand, Facebook.com redirects you to www.facebook.com. You can either browse directly to the redirected site (e.g. twitter.com or www.facebook.com), or you can add both variations (e.g. both facebook.com and www.facebook.com) to your "Allow" list.'
good job you can have up to nine exceptions then 
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'20) I've allowed a specific website but when I go to it, I still can't see all of it - some text appears but not the whole site.
'This is because the page you have browsed to is also pulling information from another site which is still blocked by Parental Controls at the time in question. To see the full website you will have to allow access to the other blocked site. If you wish to see which sites are being used by the page in question, right click on the page you have browsed to and select "View Source". If necessary, select the Notepad to view the file. You will see a long list of text. The name of the blocked site will be listed somewhere in there. If you still wish to allow this site, copy this site name and add this into your specific website list.'
So, these parents who are not tech-savvy enough and/or don't care enough to use the perfectly good client-side parental filters are suddenly expected to go delving into source code to find out where their embedded page content is being sourced from? Or are they more likely to just switch the bloody thing off?
These are just a few of the reasons TalkTalk's Magic Button is not the solution to the problem.
Here's what I would propose (from earlier in the thread) - I think we need to give parents better education about the dangers their children may encounter online and what they can do about it. I'd like to see regular internet safety sessions for parents in all schools, focussing not only on porn but on the whole range of risks - violent material, disclosure of personal info, bullying, grooming, pro-suicide and pro-ana sites, illegal downloads, ID theft ... I'd also like to see effective, easy to use, configurable, free client-side software made widely available, perhaps installed by default on new machines and/or installed as part of internet connection.