An “ancient” Roman wall painting once owned by the 18th-century collector, connoisseur, wit and gossip Horace Walpole has resurfaced almost 200 years after his treasures were dispersed at auction.

Walpole believed his panel of Roman plaster, painted with delicate swags of leaves and flowers, a relief-moulded, garlanded head, and a banquet scene with a sprawled river god, was the real deal. He placed it over the door to his private library in the cottage to which he retreated when his famous house, Strawberry Hill in west London, was over-run with visitors. “I keep an inn at the sign of the gothic castle,” Walpole once grumbled to a friend.

Like many of his treasures, the painting was not quite what Walpole had hoped. Although it is expected to attract far more than the seven guineas it fetched in 1842, it has a low estimate of £15,000 for the auction at Sotheby’s on 3 July, and it would be even lower without the Walpole connection.

Florent Heintz, the head of the ancient sculpture department at Sotheby’s, first saw the panel when it was brought into the New York office on behalf of an American collector.

Heintz could see at a glance that most of it was 18th-century over-painting, done by one of the ingenious workshops in Rome which churned out antiquities for gullible Grand Tour collectors. It was so heavily restored that it was of minimal interest and he turned it down.

Then he met Silvia Davoli, an art historian who is tracking Walpole’s lost collection to reunite the treasures for an exhibition at Strawberry Hill next winter.

She and Heintz, who has a photographic memory and spends his spare time poring over old sale and exhibition catalogues, traced a marble sarcophagus on lions’ feet which once stood in Walpole’s garden and is now in the collection of the Fitzwilliam Museum in Cambridge.

When Davoli sent Heintz a 1745 engraving of the Roman painting made for Conyers Middleton, an English collector, who sold it to Walpole, he instantly recognised the spurned panel. “There was a delicate conversation with the owner,” he said, “when I had to phone and say: ‘Actually, I may have been too hasty.’”

Under raking light, x-ray and other tests, the panel proved to be as genuinely ancient Roman as a snow globe Colosseum. The plaster was genuine but extensively repaired, the leaves and flowers were painted in oils over the faint marks of the original decoration, and the entire lower scene had been added.

Indeed, Heintz tracked an engraving of a genuine Roman tomb painting discovered in the early 18th century, which shows half of the scene that was almost certainly copied on to the Walpole panel. He concedes that it’s an interesting early Grand Tour souvenir, since the trade was at its height much later in the century.

Walpole collected genuine masterpieces for his father, Robert, the first prime minister, at Houghton Hall in Norfolk, but as a youngest son he never had enough money to collect on that scale himself.

He filled Strawberry Hill with wonders he believed were once owned by kings, cardinals and emperors, such as a comb that he believed Pope Gregory gave to Saint Bertha.

The wall panel is now precious to Heintz and Davoli, not for the surviving scraps of real ancient Roman paint, but because Walpole once owned it.