I know of this, but as it's former incarnation, Davenport Lyons, before they handed it over to ACS.
I myself received such a letter and found lots of holes in it, so I fought it and DL stopped chasing me.
Using my IP address as proof was laughable, could have been anybody.
Then they asked me for my personal passwords and I refused, because they were using criminal charges procedure in a civil charge case.
So many things were just off and I turned out that i was right suspecting it was a money making scheme.
It even featured on Watchdog, where an elderly couple received such a letter, claiming they downloaded hardcore gay porn.
If anybody gets one of these things in the post, don't bin it, but reply to them and tell them kindly that you refuse to pay.