The team faces a daunting challenge as they search for lost masterpieces in Britain's public art collections. More than 100,000 oil paintings have been photographed and posted online as part of the Your Paintings initiative - 17,000 of them listed as "artist unknown. " Philip and his colleague Bendor Grosvenor believe they've identified several missing works by artist Thomas Gainsborough. Can they prove it? A handsome portrait of Joseph Gape, mayor of St. Albans in the 18th century, languishes in a backroom of the city's museum. The identity of the artist who painted it is unknown - but Bendor thinks it's a Gainsborough that dates back to his days as Britain's foremost high society portrait painter. The team thinks there are telling signs in the way the man is dressed and the unusual shape of the frame - but they'll need to convince Hugh Belsey, the world's leading Gainsborough expert. An even tougher challenge is posed by "Imaginary Landscape," held in London's Courtauld Institute. Philip thinks it's a rare, late Gainsborough work, painted when the artist was experimenting with dreamlike scenes. But would Gainsborough really have executed it on paper rather than canvas, and why is conservationist Aviva Burnstock troubled by a distinctive blue pigment? Philip made his name in the art world with his Gainsborough discoveries and his reputation is on the line as decision time looms.