Martin P-L writes:

1686 born
1703 matriculated at Christchurch, Oxon., aged 15.
1709 called to bar of Lincoln's Inn.
1718 married Katherine, dau. & heiress of Thos. Ravenscroft of Broadlane, Flint.
1720 died, eleven months after his father. In his will he ordered that a monument should be erected to his father, but none should be raised to himself (as is recorded on the monument itself).
1721 his wife died, aged 26.
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A note on his birth:

In 1718 he had a licence to marry and was described as “Thomas Powys of Lincoln’s (?) Inne in Middx aged 36 years and a batchelor”.  This would make him born in 1682, fours years earlier than otherwise recorded.

But his parents did not marry until 1685 so an earlier birth would have been unlikely in the social climate of those times.

Oswald Barron in his account of the family for the Victoria County History of Northants, p. 260, says he was born in 1687.

Is there any MI at Lilford or Achurch that gives his birth?  Yes, there is a MI for him, according to Thomas Barron.

We could do with some better evidences.

The MI says he died in March 1719/20 aged 32.  So he was born in 1687/88.  (See full details in Mon Inscripts Notes.)

See below.
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Searching for his birth, I found this at St Nicholas, Deptford, Kent on the Register page:

“Baptised 1686
.  .  .  .
Fabruary
.  .  .  .
28  Tho: s. of Tho: Powis”

And at St James, Piccadilly there was in March 1689/90`:

“24  Thomas Powis of Joseph and Grace,  b. Feb 24”
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TFPL, Jan 2021: The above was based on an assumption that his birth year was 1686.  In fact the memorial to his father included a mention that this Thomas died on 3rd of March 1719/20 aged 32.  So he was born somewhere between 3rd March 1886/87 and 2nd March 1687/88.

The other information we have on his birth is that he went to Oxford University on 8 Nov 1703 and was then aged 15 (not uncommon to go to university at that age in those times).  So he was born between 8th November 1687 and 7th Nov 1688.

If both these ranges are true he was born between 8th November 1687 and 2nd March 1687/88, say Jan 1687-8 plus or minus two months.  Corroborating this are the ages at which his two brothers went to Oxford.  In both cases their age at entry agrees to the year with their birth date given at baptism.  In addition, for his brother Ambrose, the  third son, his age on his memorial at Thorpe Achurch agrees exactly with his birth date from his baptism record.

But still no baptism registration can be found.
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From Lincoln’s Inn, Vol I, Admission from A.D. 1420 to A.D. 1799, pub 1896, p. 360:

“1701 July 12  Folio 72b.  THOMAS POWYS, son and heir app, of Sir Thos. P., of Lincoln’s Inn, knt., a master of the bench.”

He would have been 14 at that time.

and

From Lincoln’s Inn, the Black Book, vol III, pub 1899 p. 235:

“Call to the Bar, July 1st, 1709: Thomas Powys and George Hadley.   fo. 174”

He would have been 21 and around four months at that time.
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TFPL, Feb 2021: From Foster’s Alumni Oxoniensis, 1500-1714, vol 3, p. 1197:

Powys, Thomas, 1sts. Tho., of London, equities. CHRIST CHURCH, matric. 8 Nov., 1703, aged 15; of Lilford, Northants, bar-at-law, Lincoln’s Inn 1709; died 3 March 1719-20, aged 32; grandfather of Thomas, Baron Lilford.  See Foster’s Judges and Barristers.
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His burial transcription:

First name(s)                       Thomas
Last name                           Powys
Status                              Son
Death year                          1719
Burial year                         1719-20
Burial date                         09 Mar 1719
Parish                              St Peter
Place                               Lilford
Denomination                        Anglican
Residence                           NTH   (Not this Parish?)
Relative's details                  Tho: Sr
County                              Northamptonshire
Country                             England
Notes                               esq, eldest son
Extra text                          Relation details.
Repository                          Northamptonshire Archives
Event type                          Burial
Burial unique reference identifier  B0536
Archive reference                   194p/1
Record set                          Northamptonshire Burials
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His baptism has now been found.

Curiously this was in the same parish as all the other children of Sir Thomas and Sarah.  Ancestry has what appears to be a full set of images of all the pages of St Giles in the Fields, Holborn parish register(s).  But still no search for his entry can reveal it in the index.  So what I did was to start with his next sibling, Sarah who was born around a year after him, and then work backwards through the pages from a sensible time before her conception in February 1688.  This fairly quickly brought up this elaborate entry for his baptism on 22nd of December 1687:

“Thomas Son of Sr. Thomas Powis Knight his Majties. Attorney Genr’ll. and Sarah his Wife was born on the 21th of Decembr. being Sr. Thomas’ first Son and Christened the day after.  Viz: on the 22th 1687.”

The use of the, to us, odd form of 21th and 22th could be because they said “one and twentieth day of” and “two and twentieth day of”.

Notice that this is yet another case of the family surname being spelled ‘Powis’.
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His burial entry is found in the Bishop’s Transcript of the burials register for Lilford church:

“A True Coppy of ye Register Book of Lilford, for 1719
“. . . .
“Thomas Powys Esqr eldest Son of Sr Tho: was buried Mar:9 1719.”

This burial was clearly eleven months after his father’s, in 1719/20, at the top of the same page.
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Will of Thomas Powys of Lilford, Northamptonshire 14 March 1720 PROB 11/573

Frome the Northants FHS bookllet of Mon Inscriptions for St John the Baptist, Thorpe Achurch, pub 2015:

at the end of his father’s Monumental inscription there is this:

“THOMAS POWYS of Lilford Esqr eldest son of Sr THOMAS POWYS Kt succeeded his father, as in estate, so in virtue.  He married CATHERINE daughter and co-heir of THOs RAVENSCROFT of Broadlane in the County of Flint Esqr and by her left one son named, as his father and grandfather THOMAS.  As by his last will and testament his piety ordered that this monument should be sacred to the ashes of his father.  His humility forbad that any other memorial should be raised to himself.  He dyed the 3rd of March 1719/20 aged 32.”
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The Licence for marriage issued on 26th April 1718 has;

“... Thomas Powys of Lincoln’s (?) Inne in Middx aged 36 years and a batchelor ... intended ... to maryy Catherine Ravenscroft of parish of St Giles in the feild [sic] ... Middx aged above 23 years and a spinster. ... licence for them to be married in the parish church of St Anne Westminster.”

Signed: Thomas Powys.
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And this marriage transcription looks like theirs:

First name(s)           Thos
Last name               Powis
Marriage year           1718
Parish                  St Anne, Soho
Spouse's first name(s)  Katherine
Spouse's last name      Ravenscroft
County                  Middlesex
Country                 England
Archive                 City of Westminster Archives Centre
Record set              Westminster Marriages
Category                Birth, Marriage, Death & Parish Records

FindMyPast claims that an image goes with this transcript, but the image is no longer shown so we do not know the actual date.
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TFPL, Feb 2021: a copy of the image has been received from the City of Westminster archives for St Anne’s Soho and the entry is:

[St Anne’s, Soho Marriage Register]

“April 1718
. . . .
28 Thos PowIs & Kather~ Ravenscroft   . . .   per LIC”
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