A Warning to the Curious (1972, TV)

"An amateur archaeologist travels to the coastal Norfolk town of Seaburgh. His research has uncovered an old Anglo-Saxon legend telling of three fabled crowns that protect England from invasion. One is said to have been lost in a war, a second lost to the sea - but the third is thought to be hidden somewhere near Seaburgh. He checks into a local inn where, despite the suspicions of the locals, he hears that another archaeologist searching for the crown was found dead more than a decade ago. His inquiries lead him to the grave of a local man, William Ager who was said to be the last of his line and a guardian of the crown. He goes to search the woods near Ager's former home but his excavations will unearth something more than the fabled crown. " via IMDB

Based on the story by M.R. James.