They have rapidly become one of the Royal Family’s greatest assets and so it proved again last week when the Duke and Duchess of Edinburgh stepped up to the plate and carried out a historic ceremony on behalf of King Charles. But as Prince Edward, 60, and Sophie, 59, watched French soldiers parade in the Buckingham Palace forecourt to mark 120 years of friendly relations between the two countries the couple must have reflected that their rise has been many years in the making.Their ascension, former BBC royal correspondent Jennie Bond notes, is also setting the perfect example for two young royals of how to make a success of being the 'spare' to the throne.