Warner Bros is set to cast a £100m spell on the Leavesdon Studios near Watford, when it buys and redevelops the home of the Harry Potter film franchise. The Hollywood giant is this week set to unveil plans for adding two new sound stages to house a Harry Potter set tour, plus a revamp of the site’s visual-effects and prosthetics departments. The acquisition will sustain 1,500 jobs and create 300 more.