LONDON (Reuters) - A re-discovered painting by Frans Hals, believed until recently to be a copy of the Dutch master, has sold for $14 million, nearly twice its pre-sale estimate.

The picture helped Sotheby's raise $101.5 million overall in its London old masters auction late on Wednesday, the second highest amount at an equivalent sale in the company's history.

"What was ... noticeable in tonight's sale was the extraordinary breadth of the buyer base," said Alex Bell, head of old masters for the auctioneer.

"Our sales are now attracting a more diverse and international spectrum of buyers than ever before."

Sotheby's, rival Christie's and other auction houses are hoping that the art market boom can continue despite spreading economic gloom elsewhere.

The emergence of super-rich Russians and other new buyers in India, China and the Middle East has helped spur values to record highs, and companies hope the breadth of buying will underpin a market some experts argue is over-inflated.

The Hals, called "Portrait of Willem van Heythuysen", was painted in the 1630s and for about 300 years was considered to be authentic.

But according to Sotheby's, it disappeared after 1963 before being re-discovered recently and re-instated as a genuine Hals. It had been expected to make around $8 million.

Sotheby's also sold a work by British master JMW Turner for $10.7 million, towards the lower end of estimates, and paintings from the collection of the late eccentric German philanthropist Gustav Rau made $12.3 million, over double pre-sale estimates.

Rau ran a hospital in Africa from where he would fly to London to attend auctions.

At Christie's earlier this week, a painting by French artist Jean-Antoine Watteau presumed lost and unseen by the public for almost 200 years fetched $24.4 million, three times expectations and a record for any French old master painting at auction.

Three sketches by Spanish master Goya, also presumed lost for more than 130 years, went under the hammer for $7.9 million, double the pre-sale estimate.

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