ACS Law are a spin off of Davenport Lyons. I had a letter from them sometime ago. I checked out their recently setup website and the documents it contained. These were originally produced by Davenport Lyons solicitors. The letter was not concerned with what had been downloaded but what I had allegedly uploaded as that seemed to be their main concern. They threaten all sorts of court action in legal terms and of course demand money. I politely but firmly denied having 'offered for upload' the game and told them I would not pay and settlement either now or in the future. I did not go on to outline the flaws in their reasoning ie another person using my WiFi either with or without my knowledge. In this country you are not responsible for what happens on your IP just because it is registered to you despite what they say in the letter. I have not heard from them since. To prove in an English court that you as an individual had committed an offence of copyright breach they would need to forensically examine your computer or all the computers in your home, they will not do this as it takes too long and costs too much.
My advice is that if you do receive a letter from ACS then don't ignore it, write back denying the accusation and inform them you will not be paying any settlement. Tell them you will be contacting your solicitor should they wish to proceed further and the Law Society to make a complaint if they continue to write asking for money. [They do have a code of conduct, two letters then put up or shut up]
I have not heard from them for over six months now and doubt I will.
This is not a [problem] but a legal way of generating cash from the uninformed. They do obtain our details legally through the High court presenting the minimal of evidence to get the order for the service provider to disclose our details.
If you want to avoid anyone else tracking your internet activity use a proxy such as Relakks.