The met website says to report to your local police
http://www.met.police.uk/computercrime/index.htm This will probably make things easier if it turns out the PC was used for something particularly illegal, but I doubt they will investigate anything unless you know it has been used to commit a crime.
Maybe your ISP will be able to give you some information on what traffic your IP has been involved with.
As far as I know any of the tools to monitor/track the traffic would need to be installed while the connections are taking place, so unless your planning on reconnecting the infected PC to the net then there isn't much you can do. And I assume its long since formatted now!
Rich.