Hi Jeremy Looking at the Catalogue of Meteorites by G.T.Prior and Max Hay 1953 (Harold Ridley book) I see that the Gibeon Meteorite fell in SW Africa at Great Namaqualand and the finds are well documented in this book pages 131 and 132. Many large and small masses were found over wide area and 51 large masses over 15 tons have been recovered. These specimens have found between 1838 and 1910. So it is likely that the specimen you saw at Tatton Park could well be one of those.It was classified as Iron fine octahedrite. A 1.5 lb piece was sold at Christies in February 2021 for 12.500 dollars, So the 325 lb specimen at Tatton must worth a few quid! Christies gives this description :
Details
Like most iron meteorites, Gibeon meteorites formed 4.5 billion years ago within the molten core of an asteroid whose shattered remains are part of the asteroid belt. After wandering through interplanetary space, several thousand years ago the Gibeon mass slammed into Earth’s atmosphere where it exploded and rained down over what is now the Kalahari Desert in Namibia. In previous generations, indigenous tribesmen recovered small meteorite fragments at or near the surface and fashioned them into spear points and other tools. This specimen was recovered with the aid of a metal detector. Gibeon meteorites were not known to westerners until 1836. They formed deep in the iron core of an asteroid that resided between Mars and Jupiter.