Some writers have also accused John Howard, 1st Duke of Norfolk; Margaret Beaufort, Henry VII's mother; and Jane Shore (Edward IV's mistress). Pollard writes of these theories: "None deserve serious consideration. The problem with all these accusations is that they beg the question of access to the Tower without Richard's knowledge and overlook the fact that Richard was responsible for the safekeeping of his nephews". The Beaufort theory has more recently been supported by Philippa Gregory in her BBC documentary series "The Real White Queen and her Rivals", but is not supported by any evidence other than a speculative one of possible motive. Historian David Baldwin suggests that Henry VII's reticence on the subject may have been due to the fact that at least one of the princes was still alive; he considers that the likelier candidate for survival would be Richard and that Edward may have died of a malady. Baldwin argues that it is "impossible" that no one knew what happened to the Princes after they entered the Tower; he believes Richard III and Henry VII, leading courtiers and their mother would all have known the boys' whereabouts and welfare. Baldwin argues that had this been the case, Henry VII would have had the choice of keeping quiet about the survival of Richard, or having him executed, and concluded, "He [Henry] would have been happy to let people "think" the boys had been murdered, but not to speculate when or by whose hand." During the reign of Henry VII, two individuals claimed to be Richard, Duke of York, who had somehow escaped death. Lambert Simnel initially claimed to be Richard, before changing his story and claiming to be Edward Plantagenet, 17th Earl of Warwick. Perkin Warbeck later claimed to be Richard, appearing in Ireland and calling himself king Richard IV. Margaret of York, Duchess of Burgundy, formally recognised Warbeck as Richard. Margaret, Richard III's sister, an unrelenting opponent of Henry VII, had previously recognised Simnel as Warwick. Warbeck was also accepted as Richard by James IV of Scotland. After a failed attempt to invade England he was captured. He retracted his claims, was imprisoned and later executed. Many modern historians believe he was an imposter, whose supporters accepted his claim for political reasons. The fact that two persons claimed to be Richard led the 18th-century writer Horace Walpole to argue that Richard had in fact escaped death, and that Warbeck genuinely was Richard, a view also supported by the Scottish historian Malcolm Laing.