His baptism from Family Search:

Name:                William Stedman Gillett
Gender:              Male
Christening Date:    14 Sep 1800
Christening Place:   OLD CHURCH, SAINT PANCRAS, LONDON,ENGLAND
Birth Date:          06 Aug 1800
Father's Name:       Gabriel Gillett
Mother's Name:       Mary Ann
FHL microfilm:       597,801, 597,802, 597,803, 597,804, 597,805.
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Alumni Oxoniensis says he was of Inner Temple in 1821.

From Burke's Landed gentry, c.1895.  No mention of a wife...

In 1881 census he was a 'Proprietor of land and other property.' and had a tutor for his son and 8 indoor servants including butler, footman and cook.  I wonder how many outdoor servants there were?

RCL P-L gave him a second daughter, Eliza, between Mary Hodgson G and Wm Arthur G, so she may have been married at the time of the 1881 census.  But this dau is not on the 1861 census so may more probably have died an infant.
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The 1861 census of his household was (with corrections of several transcription errors):

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

William S     Gillett  Head          Married    Male    60  1801  Proprietor Of Land, Houses shops and funds.
                                                                                      St Pancras, Middlesex
Eliza         Gillett  Wife          Married    Female  39  1822  -                   -
Mary H        Gillett  Daughter      -          Female   4  1857  -                   Kingston on Thames, Surrey
William A     Gillett  Son           -          Male     1  1860  -                   Harefield, Hampshire
Arthur K      Grant    Stepson       -          Male    11  1850  Scholar             South End, Essex
Adelaide A C  Grant    Stepdaughter  -          Female  10  1851  Scholar             Marylebone, Middlesex
Mary Ann      Gillett  Mother        Widow      Female  80  1781  Fundholder          Middlesex
John          Sims     Servant       Married    Male    52  1809  Butler              Gosport, Hampshiren
Eliza         Slater   Servant       Married    Female  26  1835  Nurse               Whitchurch,  Hampshire
Mary Ann      Butcher  Servant       Unmarried  Female  18  1843  Nurserymaid         Southampton, Hampshire
Eliza         Sims     Servant       Married    Female  50  1811  Cook                East Stour, Dorset
Jane          King     Servant       Unmarried  Female  30  1831  Lady’s Maid         Codford St Mary, Wiltshire
Elizabeth     Dedman   Servant       Unmarried  Female  24  1837  Housemaid           Hampshire
Mary Ann      Coles    Servant       Unmarried  Female  23  1838  Housemaid           Welton, Hampshire
Charlotte     Hogg     Servant       Unmarried  Female  22  1839  Kitchen Maid        Southampton, Hampshire

Census details
First name(s)                William S
Last name                    Gillett
Relationship                 Head
Marital status               Married
Gender                       Male
Age                          60
Birth year                   1801
Occupation                   Proprietor Of Land And House...
Birth town                   St Pancras
Birth town as transcribed    ST PANCRAS
Birth county                 Middlesex
Birth county as transcribed  MIDDLESEX
Birth place (other)          England
House name                   Hall Field
Street                       Beltran Road
Parish                       South Stoneham
County                       Hampshire
Country                      England
Hamlet                       Shamblehurst
Ecclesiastical district      Holy Saviour
Registration district        South Stoneham
Enumeration district         5 E
Archive reference            RG09
Piece number                 681
Folio number                 87
Page number                  16
Family member last name      SLATER, SIMS, GRANT, BUTCHER, KING, DEDMAN, COLES, HOGG, GILLETT
Family member first name(s)  ELIZA, JOHN, ADELAIDE, MARY, JANE, ELIZABETH, CHARLOTTE, ARTHUR, WILLIAM
Record set                   1861 England, Wales & Scotland Census

In houses that were part of the farm, there were also:
1. An agricultural labourer with his wife and daughter
2. A carter with his wife and four sons plus a grandson

The stepson Arthur Grant later inherited his gt-grandfather’s baronetcy though from his first cousin once removed, that male line having died out, and became Sir Arthur Grant bt.  The family survives and, apparently, still owns Monymusk House, nr Aberdeen, Scotland.

Wm S G’s mother is known in other records as Maria not Mary Ann as in this census.
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In the 1871 census his house was called "Harefield mansion" and his household was:
Himself a farmer of 167 acres with 12 men, 6 boys and 3 women, born in Surrey St Mary's Bermondsey (probably incorrect)
His wife
A son daughter and a (younger) son
A butler
A footman,
A lady's maid
A cook
Two housemaids
A nursery maid
and a Kitchen maid, thirteen in all.

In adjoining farm properties there were:
1. A coachman with his wife and family of five,
2. A farm bailiff with his wife and three children
3. Another farm bailiff with his wife and two children
4. An agricultural labourer with his wife and four children.
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At marriage he was a bachelor (ie no previous [legit?] kids) and gentleman living at Trinity rectory (St MaryleBone?); his father was Gabriel Gillett deceased; a witness at his wedding was Maria Ann Gillett, almost certainly his mother.  Also Elizabeth Gillett was a witness, not sure what relation of his she was, possibly the wife of his brother Gabriel Gillett.  Another witness was Henry Edwards who could have been a friend or relation of anyone... and was in fact the husband of his wife’s sister Maria Churchill Edwards, née Coster.
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The Inner Temple admissions database records him as admitted on 8th Jan 1821.  His address was Exeter College, Oxford.  He was the second son of his father who is down as Gabriel Gillett of Guildford St, St Pancras, London.
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In 1851 he was living, unmarried but with a household of three staff (extending to the following page), at Rose Villa, Ham Common, Surrey.  He had with him a nephew George Gillett who was a student of the Inner Temple and aged 23.  His household also had three servants living in: a butler, a housekeeper and a cook.
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2011, TFPL: His birth from the new Family Search site:

William Stedman Gillett

England Births and Christenings, 1538-1975

birth: 06 Aug 1800
christening: 14 Sep 1800 OLD CHURCH,​SAINT PANCRAS,​LONDON,​ENGLAND
residence: 1800 London,​ England

parents: Gabriel Gillett,​ Mary Ann

name: William Stedman Gillett
gender: Male
baptism/christening date: 14 Sep 1800
baptism/christening place: OLD CHURCH,SAINT PANCRAS,LONDON,ENGLAND
birth date: 06 Aug 1800
father's name: Gabriel Gillett
mother's name: Mary Ann
indexing project (batch) number: C04793-8
system origin: England-ODM
source film number:
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Two records are to be found on a search of all National and Local records office through the TNA Discovery search engine:

Gillett v Beard. Bill only. ( William Stedman Gillett v George Beard)

Court of Chancery: Six Clerks Office: Pleadings 1801-1842. Supplementary: pleadings etc sorted from the Six Clerks Office after 1842. Records sorted in 1844 and found... Gillett v Beard. Bill only. ( William Stedman Gillett v George Beard).
Held by: The National Archives - Chancery, the Wardrobe, Royal Household, Exchequer and various commissions
Date: 1840
Reference: C 13/3048/64
Subjects: Litigation

Useful Registered Design Number: 1969. Proprietor: William Stedman Gillett. Address: Woburn Place, Russell Square, London. ...

Patents, Designs and Trade Marks Office: Non-ornamental ('Useful') Designs Act 1843 Representations. Designs 1801-2000. (Described at item level). Useful Registered Design Number: 1969. Proprietor: William Stedman Gillett. Address: Woburn Place, Russell Square, London. Subject: Diaphragm to be used with a microscope. Category: Optical
Held by: The National Archives - Board of Trade and successors
Date: 20 July 1849
Reference: BT 45/10/1969
Subjects: Photography and film | Trade and commerce
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Further to the second, there is an entry in the book “The Repertory of Patent Inventions”, New Series--Vol II, July - December 1834, pub by W Simpkin and R Marshall, titled:

Specification of the patent granted to William Stedman Gillett, of Guildford Street, in the County of Middlesex, Esquire, for certain improvements to Guns and other small Arms--Sealed February 8, 1834.
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Another finding is in a case in the Courts between him as designer and cab maker against a cab proprietor for the latter’s infingement of their patent on some cab design.  In the Counsel’s opening speech, he explained: “Mr Gillett, gentlemen, is a private gentleman, a man of science, who had devoted a good deal of attention to the subject of this patent”.

According to another report:

“It soon became obvious that the cabs as originally designed by Hansom were unsuitable for London roads, and John Chapman designed numerous modifications, which he and his financing partner William Stedman Gillett patented in 1836. In particular they made the wheels much smaller, placed the driver’s seat high up at the back of the cab, and placed the doors in the side, to prevent passengers from leaving without paying, as had occurred with earlier back-door models. Chapman and Gillett then licensed their patent to the Safety Cabriolet and Two-wheel Carriage Company. Fifty of the newly-designed cabs were soon plying for hire in London, and the new vehicles were a huge success. The name 'Hansom Cab' by which they were originally known stuck, although by rights, the vehicle should have been called a 'Chapman Cab'.”
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From RCL P-L's notebook:

Arms: Ermine on a bend engrailed sable three lucies heads erased argent collared with a bar gemel gules.

Crest: A lucy's head erect and erased gules collared with a bar gemel argent.

Motto: Spes Mea In Deo

Ermine on a bend engrailed sable three lucies heads erased argent collared with a bar gemel gules.

From RCL P-L's notebook.