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This was the sum of money that Agnew’s had advised the Tate Gallery to pay for the illustrations. “Unfortunately, the Tate would appear to have shown their hand and the vendors accepted a better offer,” said Andrew Wyld of Agnew’s.

The private sale raises the possibility that the watercolours would never be seen in public and would disappear abroad, but Ms Howie, while not being drawn on the nationality of her client, said: “He is a man who understands the importance of these watercolours to scholars. Their loan to a museum has not been ruled out.”

Blake had been paid just £21 for a group of 40 illustrations for the 1808 edition of The Grave by publisher Robert Cromek, but only a dozen designs were eventually used by the engraver Luigi Schiavonetti.

The illustrations were sold by Cromek’s dependants at an Edinburgh auction in 1836 and for the next 164 years Blake scholars were unaware of their whereabouts. Then, in the spring of 2001, two Yorkshire book dealers, Paul Williams and Jeffery Bates, spotted a red morocco slipcase titled in gilt Designs for Blairs Grave, containing 19 watercolours, while browsing through a secondhand bookshop, Caledonia Books, on Glasgow’s Great Western Road.

The dealers acquired the folio and consigned it to Swindon book auctioneer Dominic Winter (for whom one of the men worked as a regional agent). They, in turn, contacted various Blake scholars, including Robin Hamlyn of Tate Britain, the gallery with a world-famous collection of Blake drawings, who were in no doubt that the watercolours were genuine.

However, Caledonia Books learnt of the planned sale of the folio at Dominic Winter in June 2002 and served a High Court writ on the two Yorkshire dealers, suing for the return of the illustrations and costs of more than £15,000.

The outcome of the case was expected to hinge on whether money was paid in receipt of the folio by Williams and Bates, or whether it was taken on approval. The case also attracted the attention of the estate which had originally sold the folio to Caledonia Books, and they joined proceedings.

Eventually the dispute was settled to the satisfaction of all parties, who signed a confidentiality agreement not to disclose the terms which led to the multi-million pound sale of the watercolours. But an Antiques Trade Gazette contact in the book trade said that the Yorkshire dealers appeared to have won most of the argument. “From what I have heard, they got the lion’s share of the settlement, while the Glasgow bookshop and their customer each got substantial amounts of money”, he said.