A battered and corroded thumb-sized piece of bronze has turned out to be a unique find, the earliest representation of an Egyptian deity from any site in Britain – and appropriately, after almost 2,000 years hidden in the ground, it is Harpocrates, the god of secrecy and silence.

The little figure was found at Silchester, site of an abandoned Roman city in Hampshire, in last summer's excavation, but his identity was only revealed in months of careful conservation work. His Greek and Roman designation as Harpocrates, the god of spymasters, is actually a transcription error.

"In Egyptian mythology the figure is known as Horus, the child of Isis and Osiris," said Professor Mike Fulford of the University of Reading, director of the Silchester excavation. "He is often shown with his finger in his mouth, a gesture that in Egypt represented the hieroglyph for his name, but was misinterpreted by the Greeks and Romans, resulting in his adoption as the god of silence and secrecy."

He was originally an ornament on an object, which is itself unique. "The figurine was attached to part of a charcoal-burning brazier which would have been used to provide heating and lighting. This brazier is the only one found in England so we are doubly excited," Fulford said. "The brazier, the sort of thing you would expect to find in Pompeii, is the first evidence of such a luxurious item from Roman Britain."

The context of the find suggests the brazier was imported, and later thrown out into a rubbish pit, in the first century AD.

Silchester is one of the most enigmatic Roman sites: after it was abandoned in the 7th century, with houses tumbled and the wells filled in, it was never reoccupied. A medieval abbey and manor farm clipped only a corner of the site; today, it remains open farmland surrounded by spectacular ruined Roman walls, still 20ft high in places.

Fulford has been digging at Silchester for half a lifetime and now returns every summer for training digs with his students and volunteers from all over the world. They are gradually peeling back the layers of an extraordinary history.

He now believes it was an iron age city of up to 10,000 people, the oldest and largest in Britain, built on the regular grid pattern which historians had believed arrived with the Romans. The evidence suggests Silchester never regained its wealth and power after the Roman invasion, and may have been burned to the ground and rebuilt in the Boudiccan rebellion of 60AD.

Among its puzzles are the dog skeletons which turned up all over the site, one found carefully buried standing upright, still on guard after 2,000 years. Other skeletons show cut marks from flaying, suggesting the inhabitants had a flourishing craft industry of making puppy-fur cloaks.

Harpocrates will be returning to his home of the last two millennia this weekend: he will be on display at Silchester as the site opens to visitors on Saturday and Sunday – complete with Roman legionaries, the Legio Secunda Augusta, who will be pitching their tents beside the site and breaking the tedium of camp chores with a little light gladiatorial combat.

Open days at Silchester mark the start of the Festival of British Archaeology, the largest event of its kind in the world. Over the next fortnight, hundreds of historic sites, excavations, archaeology stores and museums will welcomethe public, with events including re-enactments, lectures and a chance to try skills, from flint knapping to dowsing.