Speculative invoicing is the name given to the practice of identifying alleged copyright infringers (filesharers), obtaining their contact details (usually with a Norwich Pharmacal Order), and then sending letters demanding payment with the threat of potential court action.

The first successful case against P2P filesharers was Polydor v Brown 2005.

For speculative invoicing cases in the UK see:

Davenport Lyons (2007 - 2009)
ACS:Law (2009 - 2011)
Tilly Bailey Irvine (2010)
Gallant Macmillan (2010)
Golden Eye International (2011 - )

Speculative Invoicing Handbook (advice for those that have received letters. Unattributed, uploaded by TorrentFreak.com)

References