Princess Anne “would have made an amazing Queen”, a former royal staff member has said, as the Princess Royal celebrates her 74th birthday. Anne is at the centuries-old Balmoral Castle alongside other high-ranking members of the family – several of whom have issued touching public well-wishes on her big day. And the princess, long-hailed as being among the hardest-working royals, won’t be “slowing down anytime soon”, according to one of the King’s former butlers. Grant Harrold, who served the then-Prince Charles between 2004 and 2011, said: “She used to do at least double the amount of engagem…