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The 355 unframed watercolours, of which 287 are signed works by the accomplished Cheshire/Merseyside artist, were a recent attic discovery.

It appears that when the artist died he left a tin trunk containing a personal collection of his best work to a friend in nearby Knutsford. The trunk was promptly placed in the attic where, what amounted to a lifetime of work, stayed unopened for 40 years.

It was recently submitted to Sir Patrick by a local solicitor dealing with a probate matter.

Sir Patrick regards the discovery as one of the most remarkable finds of his career. "What is perhaps even more pleasing from a professional viewpoint is the quality and condition of the works," he said. "Most are small, charming rural and coastal landscapes. They show tranquil scenes of life in pre-War, horse-drawn Britain."

Interestingly, nearly all Constantine's works include two working horses in the composition, often the grey and chestnut duo who lived on the family farm near Sheffield. Similar watercolours by Constantine, a member of the Sheffield Society of Artists, have made prices of up to £1000 at auction.

The collection - plus 68 works by other well-known artists of the period including Clarkson Stanfield, Albert Goodwin, David Cox Jnr and Thomas Bush Hardy - will be sold on September 7 at St Peter's Assembly Rooms in Hale, Cheshire.

Meanwhile, a watercolour of The Ruins of Hatfield Castle near Doncaster by Joseph Mallord William Turner (1775-1851) has been unearthed by Cambridge auctioneers Cheffins while conducting a valuation at Willett House, Somerset.

The watercolour, measuring 9 x 13in (23 x 33cm) was previously only known from an engraving of 1852. It was painted c.1797, a date which coincides with one of Turner's visits to Yorkshire.

In a private collection for the last 50 years, it will carry an estimate of £7000-10,000 as part of Cheffins' two-day Fine Art and Country House sale on September 26-27.