The Duchess of Cambridge joined Princes William and Harry for a wizard day out at the Harry Potter film studios yesterday.The Duke and Duchess of Cambridge and Prince Harry were attending the inauguration of the Warner Bros. Studios in Leavesden, Hertfordshire, where the highlights of their visit include a trip to the Great Hall, a special effects demonstration on Diagon Alley and even a wand lesson in The Gryffindor Common Room.Kate, who is due to give birth in July, wore a short £38 spotted dress by Topshop and black suede heels with a black Ralph Lauren jacket pushed up at the sleeves.