Her baptism (no birth reg found):

“                             Page 182
“BAPTISMS solemnized in the Parish of St Paul’s Knightsbridge
“  in the County of Middlesex in the Year 1851
“. . . .
“When Baptised:       1851 June 25   (it looked like ’1857’ on the register, but it wasn’t)
“No:                  1451
“Child’s Forename:    Eleanor Mary Agnes
“Parents’ Forenames:  William Bruce & Caroline Harriet
“Parents’ surname:    Stopford
“Abode:               58 Eaton Place
“Profession:          Esquire
“Celebrant:           Robt Liddell
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Banns were published in Kent for Lionel John William Fletcher of this parish and Eleanor Mary Agnes Stopford Sackville of the parish of Lowick, Northants on 10th, 17th and 24th August 1879.

The marriage followed in Northants but only the civil record has been found.  Not true: Ancestry had a copy of the Marriage Register and it has been transcribed and entered.
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In 1881 (at last) they had one son and were at Hollington Park Cott, Hollington, Battle, Sussex:

First name(s)  Last name  Relationship  Marital status  Sex  Age  Birth year  Occupation  Birth place
Lionel    Fletcher  Head  M  M  35  1846  Magistrate b.a.cambridge  Ulcombe, Kent
Eleanor   Fletcher  Wife  M  F  29  1852  -                         St George's, Middlesex
Lancelot  Fletcher  Son   S  M   0  1881  -                         St George's, Middlesex
Emma      Collins   Serv  S  F  24  1857  Nurse                     Dunmow, Essex
Susan     Knight    Serv  S  F  33  1848  Cook                      Reading, Berkshire
Emma      Mitchell  Serv  S  F  21  1860  House & parlour maid      Moordown, Hampshire

Ref:
Archive reference  RG11
Piece number       1031
Folio              87
Page               10
Combined:          1881 ref: RG11, piece 1031, folio 87, p. 10
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IOn 1891 they had four sons and were at Ewell Mann, The Green Road, West Farleigh, Maidstone, Kent:

First name(s)  Last name  Relationship  Marital status  Sex  Age  Birth year  Occupation  Birth place
Lionel J W   Fletcher   Head  M  M  45  1846  Magistrate & DL for Kent  Ulcombe, Kent
Eleanor M A  Fletcher   Wife  M  F  39  1852  -                         London
Lancelot S   Fletcher   Son   -  M  10  1881  -                         London
Roland S     Fletcher   Son   -  M   9  1882  -                         London
Walter J     Fletcher   Son   -  M   7  1884  -                         London
Horace W     Fletcher   Son   -  M   1  1890  -                         West Farleigh, Kent
Alfred       Courtney   Serv  S  M  31  1860  Footman                   Kington, Herefords
Emma C       Cooper     Serv  S  F  20  1871  Ladysmaid                 Greenwich, Kent
Sarah        Thomas     Serv  S  F  55  1836  Cook                      Coton in the Elms, Derbys
Catherine    Mays       Serv  S  F  24  1867  Nurse                     Gloucestershire
Ellen H      Cheeseman  Serv  S  F  28  1863  Housemaid                 Ashford, Kent
Ellen S      Longhurst  Serv  S  F  19  1872  Housemaid                 Bexley, Kent
Prudence P   Copcutt    Serv  S  F  19  1872  Kitchenmaid               Aylesbury, Bucks
Gertrude M   Edlin      Gnss  S  F  28  1863  Governess                 London, Middlesex

Ref:
Archive reference  RG12
Piece number       685
Folio              16
Page               7
Combined:          1891 ref: RG12, piece 685, folio 16, p. 7      
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In 1901 they were at Ewell Manor, West Farleigh, Maidstone, Kent:

First name(s)  Last name  Relationship  Marital status  Sex  Age  Birth year  Occupation  Birth place
Lionel J W   Fletcher  Head  M  M  55  1846  Living on own means  Ulcombe, Kent
Eleanor M A  Fletcher  Wife  M  F  49  1852  -                    London
Launcelot S  Fletcher  Son   S  M  20  1881  -                    London
Lilian F P   Gereway   Vstr  S  F  34  1867  -                    Yorkshire
Priscilla    Brooks    Serv  S  F  34  1867  Cook                 Burham, Kent
Bessie       Moring    Serv  S  F  26  1875  Parlour maid         Rochester, Kent
Ellen        Quarman   Serv  S  F  27  1874  House maid           India
Rose         Weller    Serv  S  F  16  1885  House maid           West Farleigh, Kent
Annie        Swale     Serv  S  F  16  1885  Kitchen maid         Northamptonshire

Ref;
Archive reference  RG13
Piece number       763
Folio              7
Page               6
Combined:          1901 ref: RG13, piece 763, folio 7, p. 6
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Their five children’s registrations were:

Name:                               Mother's Maiden Surname:
FLETCHER, LANCELOT SACKVILLE        STOPFORD SACKVILLE  
GRO Reference: 1881  M Quarter in ST GEORGE HANOVER SQUARE  Volume 01A  Page 444
FLETCHER, ROLAND SACKVILLE          STOPFORD SACKVILLE  
GRO Reference: 1882  J Quarter in ST GEORGE HANOVER SQUARE  Volume 01A  Page 411
FLETCHER, WALTER JOHN               STOPFORD-SACKSILLE  [sic]
GRO Reference: 1883  S Quarter in CHELSEA  Volume 01A  Page 368
FLETCHER, MARION FRANCES            STOPFORD SACKVILLE  
GRO Reference: 1886  D Quarter in MAIDSTONE  Volume 02A  Page 688
FLETCHER, HORACE WILLIAM            STOPFORD SACKVILLE  
GRO Reference: 1889  D Quarter in MAIDSTONE  Volume 02A  Page 714

and:

Name:                         Age at Death (in years):  
FLETCHER, MARION FRANCES      1  
GRO Reference: 1888  M Quarter in MAIDSTONE  Volume 02A  Page 455
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In 1911 she was with her sister Fanny Louisa at the Bishop’s Palace; see Fanny Louisa’s transcript.  Again, as with all other censuses, she said she was born in 1852 so I decided to see what the record said.  In fact it was an honest mis-transcript.  It looked like her baptism was dated 1857, but the ones before and after were clearly all 1851.  So I have amended the above transcript of her baptism.  It accounts for why I could not find her in all the censuses - and wasted some time thereby.
_____________________

I was then able to immediately find her death, in 1943.  So she would have been in the 1939 Register.  She was and that gave her birth date.  And she was still at Elmscroft where her husband had died and was now with her eldest son and his wife and possibly some infants at Elmscroft, Kettle Corner, Maidstone R.D., Kent:

First name(s)  Last name(s)  Birth date  Sex  Occupation  Marital status  Schedule  Schedule sub number
Eleanor M A  Fletcher  12 May 1851  F  Incapacitated              W  122  1
Lancelot S   Fletcher  16 Jan 1881  M  Barrister At Law & Farmer  M  122  2
Margery J    Fletcher  25 Nov 1891  F  None Household Duties      M  122  3
Margery K K  Turner    04 Jun 1906  F  Cook                       S  122  4
Bertha V     Bennett   04 Feb 1925  F  Between Maid               S  122  5
The record for this person is officially closed.
The record for this person is officially closed.
Peter H      Walker    28 Mar 1932  M  At School                  S  122  8
Keith M      Walker    28 Mar 1932  M  At School                  S  122  9

1939 ref: RG101/1865E/014/22
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Her death index:

First name(s)  Eleanor M A
Last name      Fletcher
Gender         Female
Birth year     1851
Age            92
Death quarter  4
Death year     1943
District       Maidstone
County         Kent
Volume         2A
Page           1589
Country        England
Record set     England & Wales Deaths 1837-2007
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Her probate index:

1944
FLETCHER Eleanor Mary Agnes of Elmscroft West Farleigh Maidstone widow died 10 December 1943  Probate Llandudno 21 December to Lancelot Sackville Fletcher barrister-at-law.
Effects £685 192. 4d.

Sounds like her husband and she managed to live in style with no money!

Her will has been ordered in case she includes some interesting relations.
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Their marriage index:

First name(s)     Eleanor Mary A B
Last name         Sackville
Marriage quarter  3
Marriage year     1879
Marriage Finder   Eleanor Mary A B Sackville married one of these people
                  Lionel John Fletcher, Cleanor M Stopford-Sackville
District          Thrapston
County            Northamptonshire
Country           England
Volume            3B
Page number       303
Record set        England & Wales Marriages 1837-2005
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And as almost ever, Ancestry have an image of the Marriage register, transcript by me:

“                           Page 49
“1879 Marriage solemnized after Banns in the Church of Lowick in the County of Northampton
“NO:
When Married:

                        His                            Hers
                        ===                            ====
“Name:          Lionel John William Fletcher  Eleanor Mary Agnes Stopford-Sackville
“Age:           full age                      full age
“Condition:     Bachelor                      Spinster
“Profession:    Gentleman                     -
“Residence:     Yalding                       Lowick
“Father:        Edward Charles Fletcher       William Bruce Stopford Sackville
“Father’s job:  Major General (deceased)      Esquire  (deceade)

“Married in the Parish Church according to the Rites and Ceremonies of the Church of England
“   by me, Augustus Legge
“This Marriage was solemnized between us, Lionel J W Fletcher, Eleanor Mary Stopford Sackville
“In the presence of us, L H Stopford Sackville,  Lilford,  Frances Fletcher,
                        Margaret Stopford Sackville, S G Stopford Sackville,
                        Charles E Fletcher
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