Lely ‘sleeper’ – £19,000 at Duke’s.

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It was catalogued at Duke's sale last month as Follower of Sir Peter Lely (1618-1680). Provenance: Ingestre Hall, Staffordshire, formerly home of the Earls of Shrewsbury and thence by descent. Estimate £400-800.

A decent illustration accompanied this straightforward description and as soon as he saw it, Mayfair dealer Fergus Hall thought that under the layers of varnish lay an autograph work by Lely himself.

Among others who saw the catalogue for the September 27-28 sale and reached the same conclusion were Mr Hall's near neighbour in Dover Street, the Sleeper-Finder General, Philip Mould, and Sunday Times art critic Waldemar Januszczak - both, like Mr Hall, ardent admirers of Van Dyck and Lely.

Bidding on the phone, Mr Hall, who left a career in finance to set up Fergus Hall Master Paintings in Albemarle Street in 2008, beat them both to the portrait with a bid of £19,000.

"I felt that it could be an autograph work when I saw the catalogue," Mr Hall told ATG. "After I'd seen it properly I was sure. My restorer is delighted with the condition it's in."

Next step is to identify the sitter.

In his blog, Bendor Grosvenor, who works with Mr Mould as viewers of BBC TV's excellent Fake or Fortune will know, wrote: "This fine Lely surfaced in a country sale today - but unfortunately we were outbid. Here, we think the sitter might have been William Brouncker, the famous mathematician."

Mr Hall also thinks the 2nd Viscount Brouncker a likely candidate - the Cheltenham Art Gallery and Museum has a much larger oil of the aristocratic genius catalogued as 'by Peter Lely and studio attributed to' - but he is doing more research.

"I have no reservations about the portrait being an autograph Lely but I will be showing it to [17th century British paintings scholars] Diana Dethloff and Catharine MacLeod who are working on a full catalogue raisonné," he said.