Her baptism from Northants FHS via FindMyPast:

First name(s)  Mary
Last name      Powys
Baptism year   1805
Baptism date   22 Feb 1805
Relationship   Daughter of Thos lord Lilford & Henrietta Maria
Place          Thorpe Achurch, St James the Baptist
Description    St James the Baptist
Denomination   Anglican
County         Northamptonshire
Country        England
Archive ref    320p/2
Archive        Northamptonshire Archives
Record set     Northamptonshire Baptisms

In fact her baptism was on the same page as her brother Horatio’s and the actual text was:

“1805
. . . .
Mary Dautr. of Thos.
Lord Lilford, &
Henrietta, Maria,
his Wife baptd. Febry. 22nd.”

and below this in pencil:

“Mrs Drummond”

And I was informed that the Register was that for the church of Lilford-cum-Wigsthorp.
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Of the various Mary D deaths on FreeBMD, I cannot identify hers.

But the new online GRO index has it:

Name:                Age at Death (in years):  
DRUMMOND, MARY       79  
GRO Reference: 1883  D Quarter in TUNBRIDGE  Volume 02A  Page 361
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In 1851, their household was at Thorpe Achurch Lodge, Thorpe Achurch, Northants:

First name(s)  Last name  Relationship  Marital status  Gender  Age  Birth year  Occupation  Birth place

James          Drummond     Head  M  M  51  1800  Rector Of Pilton  East Indies
Mary           Drummond     Wife  M  F  45  1806  -                 Lilford, Northamptonshire
Mary Eleanor   Drummond     Daur  -  F  12  1839  Scholar At Home   Kent, Down
William Venn   Drummond     Son   -  M   9  1842  Scholar At Home   Middlesex, Highgate
Agnes Emi      Drummond     Daur  -  F   6  1845  Scholar At Home   Northants, Achurch
Herbert Powys  Drummond     Son   -  M   4  1847  Scholar At Home   Brighton, Sussex
Elizabeth      Smith        Serv  U  F  25  1826  Nurse             Not Known
Hannah         Bennett      Serv  U  F  25  1826  Cook              Not Known
Mary Anna      Sorthington  Serv  U  F  23  1828  House Maid        Not Known
Elizabeth      Walker       Serv  U  F  20  1831  Parlour Maid      Not Known
Mary           Andrew       Serv  U  F  14  1837  Nurse Maid        Not Known

Ref:
Archive reference  HO107
Piece number       1746
Folio              83
Page               12
Combined:          1851 ref: HO107, Piece 1746, Folio 83, p. 12
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In 1861 they were indeed at Gaulby:

First name(s) Last name Relationship Marital status  Gender  Age  Birth year  Occupation  Birth place

James      Drummond  Head  M  M  61  1800  Rector Of Gaulby  East Indies
Mary       Drummond  Wife  M  F  56  1805  Rectors Wife      -
Agnes E S  Drummond  Daur  M  F  16  1845  Rectors Daughter  Northamptonshire
Kate       Swingler  Serv  U  F  24  1837  House Servant     -
Sarah      Jenkins   Serv  U  F  26  1835  House Servant     Newtown, Montgomeryshire
John       Jenkins   Serv  -  M  18  1843  Coachman          Worthing, Shropshire

Ref:
Archive reference  RG09
Piece number       2254
Folio              48
Page number        7
Combined:          1861 Ref: RG09, Piece 2254, Folio 48, p. 7
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In 1881 their household was at 20 Mount Ephraim, Tonbridge, Kent:

First name(s) Last name Relationship Marital status  Gender  Age  Birth year  Occupation  Birth place
James            Drummond  Head M  M  81  1800  Hon Canon Of Peterboro Without Cure Of Souls
                                                                        India
Mary             Drummond  Wife M  F  77  1804  -                      London
Ayrksling Ponys  Drummond  Daur  S  F  36  1845  -                      Achurch, Northants
Emma             Bird      Serv  S  F  26  1855  Ladies Maid            Calby, Leicestershire

Ref:
Archive reference  RG11
Piece number       915
Folio              60
Page               3
Combined:          1881 ref: RG11, piece 915, folio 60, p. 3
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Their son Herbert Powys Drummond had his probate recorded on 1 Sept 1884, after he died on 14 Dec 1877 in Cannamore, Madras, East India.
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No probate entry found for her from 1881 to 1900.
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Their daughter Agnes Emily Powys Drummond was found also in the 1891, 1901 and 1911 censuses.  She died in 1926, on 9 Nov 1926 and probate was granted on 9 March 1927. Baptized at Titchmarsh on Oct 27 1844.

Mary and James had at least seven children.
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Her probable children from FindMyPast:

   Last name  First name(s)  Year Of Birth  Year Of Death  Year  Record set  Location

1. Drummond  Mary Eleanor  1838  —  1838  Middlesex Baptisms  Highgate, St Michael, Middlesex
2. Drummond  Herbert Powye  —    —  1847  England Births & Baptisms 1538-1975  Brighton, Sussex

And from Ancestry:

1. Name:        James Hoolbrooke Drummond
Gender:         Male
Record Type:    Christening (Baptism)
Baptism Date:   17 Oct 1833
Baptism Place:  Downe, Kent, England
Father:         James Drummond
Mother:         Mary Drummond

2. Name:        Elizabeth Henrietta Drummond
Gender:         Female
Record Type:    Christening (Baptism)
Baptism Date:   19 May 1832
Baptism Place:  Downe, Kent, England
Father:         James Drummond
Mother:         Mary Drummond

3. Name:Name:   William Lyttleton Rowys Drummond  or ‘Powys’
Gender:         Male
Record Type:    Christening (Baptism)
Baptism Date    21 Dec 1834
Baptism Place:  Down, Kent, England
Father:         James Drummond
Mother:         Mary Drummond

4. NAME:        William Vern Drummond  + image of register
GENDER:         Male
RECORD TYPE:    Baptism
BAPTISM DATE:   13 Jan 1842
BAPTISM PLACE:  St Michael, Highgate, Camden, England
FATHER:         James Drummond
MOTHER:         Mary Drummond
REGISTER TYPE:  Parish Register

5. Name:        Henry Albertson Drummond  or ‘Atherton’?
Gender:         Male
Record Type:    Christening (Baptism)
Baptism Date:   9 Apr 1831
Baptism Place:  Downe, Kent, England
Father:         James Drummond
Mother:         Mary Drummond

6. Name:        Agnes Emily Powys Drumond or Drummond?  + image of register
Age:            0
Birth Date:     Abt 1844
Baptism Date:   27 Oct 1844
Baptism Place:  Titchmarsh, Northamptonshire, England
Register Type:  Bishops Transcripts

A total of 8 children

The order of births is:

                                 Downe      Other       Image
1831  Henry Albertson              Y                     N  (order started)
1832  Elizabeth Henrietta          Y                     N  (order started)
1833  James Hoolbrooke             Y                     N  (order started)
1834  William Lyttleton Rowys      Y                     N  (order started)
1838  Mary Eleanor                 N        Highgate     Y
1842  William Vern Drummond        N        Highgate     Y
1844  Agnes Emily Powys            N        Titchmarsh   Y
1847  Drummond  Herbert Powys      N        Brighton     N  (to be ordered)

Down Kent is in the north-west of the County and its registers are held at:
Medway Archives and Local Studies Centre.
Contact Information: Medway Archives and Local Studies Centre
Civic Centre
Strood
Rochester
Kent ME2 4AU
Phone: (01634) 332714
Email: malsc@medway.gov.uk
Wed:  https://www.medway.gov.uk/archives

And the Downe registers appear actually to be at:
Downe Archives and Local Studies
High Street,
Downe,
Kent,
BR1 1EX
Phone +44 (0)208 461 7070
Fax +44 (0)208 313 9975
email: localstudies.library@Downe.gov.uk

See: <http://www.kent-opc.org/Parishes/Downe.html>;
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Their marriage licence transcript:

First name(s) MARY
Last name POWYS
Licence year 1830
Licence date 07 May 1830
New calender date 07 May 1830
Bride's first name(s) Mary
Bride's last name POWYS
Groom's first name(s) J
Groom's last name DRUMMOND
Place Dioceses of England & Wales
Record set Faculty Office Marriage Licences

From Society of Genealogists who can provide a scanned copy of the original.
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Their marriage transcript:

First name(s) Mary
Last name Powys
Marriage year 1830
Marriage date 1830
Marriage place Down
Spouse's first name(s) James
Spouse's last name Drummond
Residence Down, Kent, England
County Kent
Country England
Record set England Marriages 1538-1973
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