An expert has told of her disappointment after she saw a solid gold chain named the "Coleridge Collar" which is now the centre of a High Court valuation dispute.

Marian Campbell, who was shown the heirloom in January 2007 when she was senior curator of metalwork at London's Victoria and Albert Museum, said she rated it as "one star", she told the court.

At the time she noted that it was "rather a wonderful but not medieval collar" and that part of it looked "pure arts and crafts", with other parts no older than 17th or 18th century.

Ms Campbell was giving evidence in a damages action brought by 74-year-old Lord Coleridge, the fifth Baron of Ottery St Mary, against auction house Sotheby's, which denies liability.

He says he would not have sold the collar privately in 2006 for £35,000 but for the "snap judgment" of Sotheby's expert Elizabeth Mitchell, who said it dated from the late 17th century.

He believes that the collar, which later went under the hammer at Christie's in 2008 for £260,000, is the only surviving complete Tudor chain of office and a unique national treasure.

His counsel, Joshua Munro, has told Judge Mark Pelling QC that Sotheby's were careless to say, without caveat, in June 2005 that the chain definitely dated from the late 17th century.

He claims its metal content should have been tested at the London Assay Office, and that Sotheby's should not have discounted the long-standing tradition that it dated back to the reign of Henry VII and was worn by Sir Edward Coke before being passed down the line of Chief Justices.

But Sotheby's counsel Richard Edwards says that the case for a Tudor date does not stand up to analysis and that the evidence on which it was based was "full of errors, inconsistencies and misunderstandings".

Even if a plausible case could be made for an earlier date, which was not accepted, Ms Mitchell's opinion was formed honestly and with at least reasonable skill and care.

She had approached her task in an entirely professional, conventional way, had examined the Collar carefully, read around the subject and investigated the pictorial record.

"It is clear from Ms Mitchell's evidence that she thought about the collar a great deal and would dearly have liked to be able to say that it was Tudor. Sadly, she did not feel able to do so."

Ms Campbell told the court that her "hopes were high" before the 10-minute inspection of the collar as it sounded a very exciting piece.

But, she was "troubled" when she saw it and felt some disappointment because she thought it could have been of two or three different dates – or more.

She said: "It is not as astonishing as you might suppose to form a view fairly quickly as to whether something is three-star or one-star, and that was my view at that time."

The hearing continues.