Paul Burrell, former butler of the late Princess Diana, is under suspicion for his alleged role in the disappearance of items missing from Diana's estate, but Burrell "wouldn't betray anyone's trust for any amount of money," his father told PEOPLE in its new issue. Scotland Yard detectives paid Burrell, 42, a surprise visit at his home in Cheshire, England recently, to question him about the missing items of Diana's that include a jewel-encrusted model of an Arab dhow (a ship) that purportedly went on sale after Diana's death. Police had questioned two others last November about the disappearance of the dhow -- Harold Brown, another former butler, and London fine art dealer Jan Havlik. "We are very surprised," said a spokesman for Diana's brother, Earl Spencer. "He swears on his life he knows nothing about that ship," Burrell Sr., 66, said of his son, whom Diana called "my rock" and who spurned million-dollar offers to write a tell-all book after Diana's 1997 death. By law, British police can't speak about the situation, and the Palace has withheld comment.