The answer to the question: What was the name of the sorcerer who the medallion belong to? is inside the article: During the Blitz, Charlie, Carrie, and Paul are evacuated from London to Pepperinge Eye, where they are placed in the reluctant care of Miss Eglantine Price, who agrees to the arrangement temporarily. The three children attempt to run back to London, but after observing Miss Price attempting to fly on a broomstick, they change their minds. Miss Price reveals she is learning witchcraft through a correspondence school with hopes of using her spells in the British war effort, and offers the children a transportation spell in exchange for their silence. She casts the spell on a bedknob, and adds only Paul can work the spell, since he's the one who handed it to her. Later, Miss Price receives a letter from her school announcing its closure, thus preventing her from learning the final spell. She convinces Paul to use the enchanted bed to return the group to London, and locate Professor Emelius Browne. They discover Browne is actually a charismatic showman who created the course from an old book, and is surprised to learn the spells actually work for Miss Price. He gives the book to Miss Price, who is distraught to discover the final spell, "Substitutiary Locomotion," is missing. The group travels to Portobello Road to locate the rest of the book. They are approached by Swinburne, who takes them to the Bookman, who possesses the remainder of the book. They exchange their pieces, but learn only the spell was inscribed on a medallion, the Star of Astaroth, that belonged to a sorcerer of that name. The Bookman reveals the medallion may have been taken by a pack of wild animals, given anthropomorphism by Astaroth, to a remote island called Naboombu. It was said in the 17th century, a lascar claimed he saw Naboombu. The Bookman, however, does not believe the island exists, as he looked in every chart for it, until Paul confirms its existence via a storybook., can you guess it ?
Astaroth