But I didn't download anything. I'm completely innocent. What can I do?

From Digital Economy Act Info

Firstly, talk to your ISP, they should be able to tell you how they identified your connection and the time of the infringement. However, please consider that wording of the Act means there us no requirement to prove who was responsible, just the subscriber of the connection used. So it is always worth checking you have secured your network so unauthorised users are unable to use it and ensure those that are authorised (family members, flat mates, customers etc) haven't been using it to infringe copyright.