Cressida Bonas paid an emotional tribute to her “darling sister” Pandora Cooper-Key at a memorial service on Friday. “My heart is forever tied to yours,” the actress, 35, said of Pandora, a ceramicist who died in July aged 51 following a 24-year cancer battle. Bonas, a former girlfriend of Prince Harry, was joined at St Luke’s church in Chelsea, west London, by Pandora’s friends including Queen Camilla’s son, Tom Parker Bowles. Wearing a pastel-coloured trouser suit, Bonas read a moving “letter to Pandora” that she composed after her death. Their mother, Lady Mary-Gaye Curzon, also paid tribut…