Sarah Ferguson was involved in a long-running 'friends with benefits' relationship with rapper and producer Sean 'Diddy' Combs that lasted for years in the 2000s, according to royal biographer Andrew Lownie, who repeated the allegation on The Megyn Kelly Show on Wednesday. The claim, which centres on the ex-Duchess of York's private life after her separation from ex-prince Andrew, is fiercely disputed by a source close to Sarah Ferguson and has …