Was she an heiress?  She may have had a brother John but I get the impression that he dsp.

No. Her parents had at least four children:

1800: John
1805: Mary
1806: Philip
1810: Philip
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Her parents were Rev John and Elizabeth Morres, from her baptism:

First name(s)             Mary
Last name                 Morres
Birth year                1805
Birth date                28 Jan 1805
Baptism year              1805
Baptism date              24 Feb 1805
Baptism place             Nether Broughton
Denomination              Anglican
County                    Leicestershire
Country                   England
Father's first name(s)    Revd John
Mother's first name(s)    Elizabeth
Event type                Parish Registers
Archive                   Record Office for Leicestershire, Leicester & Rutland
Archive reference         DE9102/4
Page                      43
Record set                Leicestershire Baptisms
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Her burial index:

First name(s)     Mary
Last name         Morres
Age               30
Birth year        1805
Residence         nr Mathews Green
Death year        1835
Burial year       1835
Burial date       25 Jun 1835
Church            All Saints
Place             Wokingham
County            Berkshire
Country           England
Record set        Berkshire Burial Index
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There is a grave with a family tombstone on it in the Cemetery of All Saint’s church, Wokingham and listing these on it:

full name age birth death relationship
Mary Morres 30 1805 1835 first name on monument
Philip Morres 31 1811 1842 brother of Mary Morres
Augusta Morres 2 1843 1845 daughter of Mary Morres
Beatrice Morres 76 1791 1867 wife of Mary Morres (2nd wife of Elliot)
Elliot Morres 90 1794 1884 unknown
Margaret Morres 1869 unknown
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Their marriage abstract

First name(s)            Elliot
Last name                Morres
Age                      -
Birth year               -
Residence                Britford, Wilts
Marriage year            1825
Marriage date            27 Oct 1825
Spouse's first name(s)   Mary
Spouse's last name       Morres
Spouse's residence       Nether Broughton
Denomination             Anglican
Place                    Nether Broughton
County                   Leicestershire
Country                  England
Event type               Parish Registers
Archive                  Record Office for Leicestershire, Leicester & Rutland
Archive reference        DE9102/7
Record set               Leicestershire Marriages
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The image of the page of the Parish register adds:

(a) The entry was No 30 in the Register
(b) He was a bachelor and she a spinster.
(c) The marriage was by Licence,
(d) The rector of the parish was a John Morres, who signed other marriages on that page,
(e) Their marriage was celebrated by John Gerard(?), Rector of Pembridge, Herefordshire,
(f) The witnesses were John Morres and Mary Morres.  The signature for John was the same as the above signatures for John the Rector.  So I presume that John the Rector and Mary were the parents of Mary the bridge.  --  False!  Mary's mother was Elizabeth Guard who must have been a relation of the celebrant.

My suspicion is that Mary and Elliot were related, albeit remotely.

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