I believe Bookbox has correctly revealed Mary's surname as Semisterr (Simister.) Further digging, excuse the pun, shows that Mary (23) is buried in the Richardson plot, a Mary Richardson born, GRO: 1840 Jun quarter, Dewsbury married James Simister, 6 Jul 1863, St Johns, Leeds, Mary's father William (shopkeeper) is interred in the same plot as Mary.
Thank you to all who contributed.
Janet