The seventh picture among the ones in the last link posted by Beth (scroll down), shows Princess Margaret wearing a pearl necklace with large diamond clasp similar to the Queen Victoria brooch QEII often wears. Lady Sarah has been photographed wearing the necklace, as well as frequently wearing the earrings. Also note the pearl ring on Princess Margaret's hand, and the large brooch.
Perhaps some of the other pictures around that one show Lady Sarah wearing the necklace with a clasp in back? I wonder whether the large clasp is also a brooch.