In proper storybook style, in a disintegrating cardboard box shoved to the very back of a drawer, in a castle where every nook and cranny is still stuffed with the possessions of generations of hoarding owners, a cache of valuable antique coins, some extremely rare, has been discovered.

Scotney Castle in Kent was left to the National Trust in 1970, and the gardens, with the ruin of a medieval moated manor house, Scotney Old Castle, have been enjoyed by visitors since then. But the Victorian mansion house, sometimes known as New Scotney Castle, only opened to the public in 2007 after the death of Betty Hussey, the last resident of the family that owned the estate since the 18th century. Since then volunteers have been scouring through attics and cellars, and opening hundreds of cupboards and drawers, carefully recording myriad family possessions from medieval documents to 20th-century account books.

The coins are the most exciting discovery so far. Most of the 186 coins are Roman, probably collected by the Victorian owner of Scotney Castle, Edward Hussey, and his son Edwy. But they range from a silver coin showing a sea turtle, made in the seventh century BC for the tiny Greek island of Aegina, to an 18th-century Welsh token, and just one forgery, a 19th-century fake of a rare coin for the Roman emperor Otho, who reigned for just three months until he killed himself in AD 69.

The fake helped identify the probable collector: in 1823 Edward’s teenage diaries carefully recorded spending between four and seven shillings on coins. He also wrote that he wanted to collect the coins of all the Roman emperors, and when the hobby was taken up in turn by his son, they almost achieved it: only two are missing from the first and second century emperors. In 1883 they were still absorbed by their hobby: Edwy’s diary records he “went to the British Museum with papa as he wanted to ask about some coins”.

Nathalie Cohen, a National Trust archaeologist, says going through every nook and cranny of the castle has been excavation rather than housekeeping, and is intrigued that Edward also wrote that he wanted to collect the coins of all the English kings and queens: “That collection might still be somewhere in the house too, just waiting for us to find it.”

Edward Hussey III had inherited Scotney on the death of his father and grandfather within a year. His mother believed both deaths were caused by “bad air”, and Hussey went on to bring in the celebrated architect Anthony Salvin to create a new house, keeping part of the medieval original as a romantic ruin.

The coins will be displayed in an exhibition opening at the house this weekend, which will also feature letters from Wallis Simpson, aunt by marriage of Betty Hussey, and Margaret Thatcher, who once rented a holiday flat there.

Inside the Collections, Scotney Castle, 4 November 2017-4 February 2018

• This article was amended on 3 November 2017 to clarify that Scotney Castle estate has been open to visitors since 1970 but that the Victorian mansion house was only opened to the public in 2007.

