Whoops posted too soon.... at the time of the Damien Greene scandal on parliamentary computers.
The scale of the problem was pretty big even then though. One of the final months there's official data for (Sept 2016) shows 32,164 attempts made in 30 days – roughly 1,070 a day. To put that in context, that's someone trying to access porn once a minute, every minute, ~18 hours a day.
mind boggling.