Not all treasure thieves tiptoe through the shells of Iraqi museums or churn up the deserts of Peru in their hunt for valuable antiquities. Nearer to home "nighthawkers" are using metal detectors and online auctions to strip rural Britain of its archaeological riches, and their illegal activities are proving every bit as destructive.

English Heritage has been so concerned about the extent of the depredation that it commissioned a study, which revealed that what was once an illicit hobby has mushroomed into a semi-professional criminal industry.

According to police, thieves have formed loosely connected networks to trade information, often in online forums, about new and vulnerable sites.