I have found the following in our National archives.  These are all estate files.  Going on your information I think that possibly the first two do not relate to your Mary Ann, as the second one says 'born in London, however the third one spells her surname as Stuart!   If you wish to order a file I suggest you do it through eggsa.org.  Hope this helps and does not add confusion!
DEPOT     NAB   SOURCE    MSCE     TYPE      LEER   VOLUME_NO 0    SYSTEM    01   
REFERENCE 17756/1932   PART      1    
DESCRIPTION          MCEWEN, JAMES GARDINER. (S/S MARY ANN, BORN STEWART).                 
STARTING  1932  ENDING    1932
  
DEPOT     NAB     SOURCE    MSCE      TYPE      LEER     SYSTEM    01  
REFERENCE 740/1960   PART      1      
DESCRIPTION          MCEWEN, MARY ANN. BORN STEWART. BORN IN LONDON, ENGLAND. PRSP     
MCEWEN, JAMES GARDINER. NO TRACE. DECEASED ESTATE. 
                  
STARTING  19600000      ENDING    19600000
DEPOT     TAB   SOURCE    MHG   TYPE      LEER   VOLUME_NO 0    SYSTEM    01   
REFERENCE 3125/43   PART      1    DESCRIPTION          GARDINER, MARY. BORN STUART. 
STARTING  19430000 ENDING    19430000  
REMARKS   PREDECEASED SPOUSE JAMES GARDINER