The Dering Roll, which dates from the last quarter of the 13th century and depicts 324 coats of arms, approximately a quarter of the English baronage during the time of Edward I.

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It spreads along an 8ft 6in (2.6m) roll of painted parchment and pictures 54 rows of arms, with six shields on each line and the knight's name above each in cursive script.

The roll begins with two of King John's illegitimate children, Richard Fitz Roy and William de Say, and focuses on knights from Sussex and Kent. It is thought Stephen of Penchester, who was Constable of Dover Castle from 1268 to 1299, commissioned it, and it was almost certainly produced in Dover between 1270 and 1280.

Carole Souter, chief executive of the National Heritage Memorial Fund, who helped secure the roll, described it as a Who's Who of medieval knights. "The Dering Roll provides a fascinating insight into the allegiances and loyalties that ruled the lives of the medieval aristocracy," she said.

In the 17th century, it was acquired by the noted antiquary and politician Sir Edward Dering (1598-1644), who probably took possession of it during his years of service as Lieutenant of Dover Castle. Adding a fascinating element, Dering erased one shield and replaced it with that of a fictional ancestor, Richard Fitz Dering, in an effort to forge his family history.

On December 4, 2007, it appeared at Sotheby's and sold for a premium-inclusive £192,500 to a private buyer who then applied for an export licence. Culture Minister Margaret Hodge subsequently placed a temporary export bar on the item.

It was awarded a starred rating by the Reviewing Committee on the Export of Works of Art and Objects of Cultural Interest, confirming it to be of particular importance and outstanding significance for the study of English heraldry.

Following the decision, the British Library embarked on a fundraising campaign, with an initial deadline of April 19, which was extended to July 19. Thanks to £100,000 from the National Heritage Memorial Fund, £40,000 from The Art Fund, £10,000 from Friends of the British Library, £10,000 from Friends of the National Libraries, and numerous donations from individual benefactors, they were able to secure it at the purchase price of £192,500 plus VAT.

The Roll is now on display in the Sir John Ritblat Gallery where it will remain for about six months, and it will subsequently be available to researchers in the British Library's Manuscripts Reading Room.

By Stephanie Harris