Complex sentence: While he has planning a heist of the South Kensington National Bank heist, Leslie stayed at the boarding house of Mary Coath where there he met her 15-year-old daughter, Mary Henrietta Coath.
Split into simpler sentences: While he has planning a heist of the South Kensington National Bank, Leslie stayed at the boarding house of Mary Coath. There he met her 15-year-old daughter, Mary Henrietta Coath.