Martin P-L writes:

1704 born at Lincoln's Inn Fields, in the house that his father left to him.  The only surviving child of Sir Thomas and Elizabeth née Meadows.  (An older Philip died in infancy.)

1719 his father died.  The children of the first marriage were grown up, and had presumably had money and land settled on them.  In any event, Philip, aged 15, was the principal beneficiary of Sir Thomas's will.  Of the £20,000 bequeathed, £12,000 was put in trust for him.  As well as the house in Lincoln's Inn Fields, he inherited (after his mother's death) the house at Sion.  The trustees were his half-brother, Ambrose, his mother, Dame Elizabeth, and his grandfather, Sir Philip Meadows.  Every time they wanted to alter the investments, they had to petition the Court of Chancery.

   NB: Sir Phillip Meadows cannot have acted as trustee as he died in 1718.

1719, June,  admitted to Lincoln's Inn, but there is no record of his being called to the bar.  (Fifteen is an astonishingly young age.)
1721 matriculated at Queen's College.
1730 married Isabella Lybbe, at Doctors Commons Church, London  (according to the Mormons, this church was St Benet's at Paul's Wharf).  They went the same day to his house at Sion, Isleworth.
1733 leased (or bought) manor of Goring  (Oxon Archives, PL  XV  b/4)
1734 Their first two children died in infancy.  The girl was buried at Isleworth, the boy (called Philip) in the chancel at Whitchurch.  This year a second PHILIP was born to them.  The day before the boy was christened, being Fri. Nov. 8, there came a brace of cock pheasants into my garden of Sion, which a servant of mine shot, and we had them for dinner the christening day.  (His memorandum book, transcribed by his great-great-grand-daughter, E.A.Powys.)  The godfathers were Sir William Twysden and Sir Philip Meadows jnr., the godmother Isabella Lybbe nee Twysden (Philip's mother in law, who also kept a memorandum book.  This was used by Mrs
Climenson when editing the Diaries.  It is now in the possession of Mrs Alison Inge, and will presumably pass to her son, John, of Green Farm House, Grafton, Marlborough.)
1735 They moved to Hardwick  (Twysden memo.)
1736 Thomas born, the future Dean of Canterbury.  Godfathers, his uncles Thomas and Ambrose; godmother his aunt Ann.
1738 Elizabeth born, but died 6 months later.  Philip and Isabella went to Isleworth for ten days (whether staying with the Merricks or at their house at Sion, I do not know).
1739 High Sheriff, Oxon.  (as had been two Lybbes before him).
1741 Richard born
1748 J.P.   Commission in P.R.O. C202/136/1
1759 started the Powys habit of leasing Hardwick.  He leased out the manors of Hardwick, Purley and Elvenden, comprising 40 houses and 1,050 acres of arable land, 70 of meadow, 30 of pasture, 850 of wood, 500 of heath; ie.2.5 thousand acres in all, some of it in Whitchurch, some in Goring,  Stoke, Checkenden, Maple Durham.
1760 paid £6.8000 for various farms, comprising about 720 acres.  (Oxon. Archives  PL 1 68ff)
1768 seems to have leased back all the Hardwick estate, for the residue of 500 years, from Rev. Gabriel of Reading.  The same year he mortgages 900 acres of coppices for £6,900 (PL XV k 2) [Much buying and selling is going on at this time, involving sums up to £10,000.  Their younger son was running up considerable debts at the same time.  Could there be a connection - and for such sums?]
1779 died.  buried at Whitchurch.  Though he erected a monument to his father in law, there seems to be none for him.

Whitchurch Burial Register:
  "1779 Sep 20   POWYS   Philip, Esqre 75"

Caroline says how good natured he was and moderate (Diaries p. 201).  He was friendly with the Duke of Marlborough, who arranged a regular gift of four deer from his estates.  (Diaries p. 143)
AP-L has portraits by Davidson or Davenport of him and Isabella.

PRO refs.
1732  Sir Littleton & Sir Thomas decd. + Philip, Chancery Proceeedings 2250/-
1743   + Isabella,
       Chanc. Proceedings  2099/7
1748  Isabella + Philip sen. & jnr.
       Ch. Pro.  1628/44
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TFPL, Jan 2021: His baptism is to be found in the Register of St Giles in the Fields, High St, Holburn, London in Jun 1704:

“18 Phillip Son of Sr Thomas Powis Knt and Elizabeth his wife born 2d instant.”

Another Powis!
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TFPL, May 2002: In one of Caroline Girle's notebooks I found a slip of paper which said:

"                                            May 1. 1739.
"To the Keepers in Woodstock Park, Stone, Ashworth, & Wyat.

"This is to order you to send to Mr Powis [sic], whenever he pleases to command them, every season, without any new Directions from me, one Brace of Bucks and another of Does.  S: Marlborough"

The "Marlborough" is indeed in a different hand and I wonder if it was in fact that gent's signature.  Anyhow it confirms the account in the diaries on p. 143 which quotes almost exactly those words but says it was signed "J Marlborough"; the then duke was in fact Charles and it is far more likely that the character concerned was "S" for signed or, in earlier times, "Sigillum", sealed; the colon after the "S" indicates an abbreviation.  The duke was but 2 years younger than Philip Powys.

The occasion of this command may have been that Philip Powys had been appointed High Sheriff of Oxford County in that year.  The duke was probably Lord Lieutenant of the County so they had to work together.
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I have in my possession a small books of Memorandums that he wrote, of where he ate, slept and any entertainment.  Almost no observations on what he was doing save for single word comments about any sport (shooting, probably) that he had.  A tad dull.

Curiously hed wrote his name inside the front cover as ‘P L: Powys’, indicating that he had adopted ‘Lybbe’ as part of his name, possibly as he was now master of the Lybbe estates.
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In his diary for 1764, Phil Powys writes against 30 December "30. My Wedding day, in the year 1730".  Presumably this included an allowance for the lost 14 days, though in fact he seems to have lost only 11 here.

In E A Powys' copy of Philip Powys's notes (her gt-gt-grandfather), he had recorded "Philip Powys was marred to Isabella Lybbe of Hardwick in Oxfordhire 19th Decr 1730".

In E A Powys' account of the family, page 69, she writes that they were married on the 19th December 1730 at Doctors Commons church and went the same day to his house at Sion Nr Isleworth.

On the English Origins site, there is a record of the Vicar General's Marriage licence Allegations in which Lybbe married Powys on 19th Dec 1730.  A full copy of the source of this has been ordered and is awaited.

In Boyd's Marriage Index, ISB LYBBE m. PHIL POWIS at LONDON (ST BENET PAUL'S WHARF) in 1730.
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The marriage licence of the 19th December 1730 has:

“Which day appeared personally Phillip Powys of Isleworth in the County of Middlesex aged twenty one years and a Batchelor and allegeth that he intends to marry with Isabella Lybbe of Hardwick in the County of Oxford aged sixteen years and a spinster by and with the consent of Isabella Lybbe of Hardwick aforesaid widow the natural and lawful mother of the said Isabella Lybbe spinster ... prayed licence for them to be married in the cathedral church of St Pauls London or the parish church or St Bennett Pauls Wharf London.

Signed Philip Powys.

“The same day appeared Personally Isabella Lybbe of Hardwicke aforesaid widow the natural and lawful mother of the said Isabella Lybbe spinster and maketh oath that she is consenting to the above intended marriage between the said Phillip Powys and Isabella Lybbe her daughter.”

Signed Isabella Lybbe.

Four things to note:

(a) the date of this licence was 19th December so it is not likely that they were married on that day, probably a few days later judging from other licences.  False!  TFPL, Dec 2009: The transcript of the marriage register in the Harleian Society volume for St Bene’t’s is quite clear that the marriage was on the 19th.  Interestingly the curate from Goring church, Mr Wooddeson, had been brought up to London to perform the ceremony.

(b) I wonder if St Bennetts was not truly the parish church of Doctors Commons, hence the naming in some documents?

(c) Isabella was sixteen and her baptism in Jan 1713, indicating a birth a day or so earlier, was of course in 1713/14 which was just under seventeen years before her marriage.  (From the 21st century I wonder what motivated her mother to get her married quite so young?  Perhaps the shortage of cash made Philip with his cash pile very attractive?)

(d) Why are the handwritings of the two paragraphs so different?
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Later in Aug 2006, TFPL: I have now found a transcript of the entry in the marriage register for St Benet’s of Paul’s Quay.  This was from the Harleian Parish register volume XXXIX for the year MDCCCCX, 1910.  The title is “The Registers of St Bene’t and St Peter, Paul’s Wharf, London” so that ‘Benet’ is short for Benedict.  The entry was:

19 Dec  Philip Powys, Esqr of Isleworth, Middx., B, and Isabella Lybbe of Hardwick, Oxon, S; by Mr Wooddeson, Cur. of Goring, Oxon.

So this spoils my theory about the Allegation and Marriage dates.  Perhaps (another good speculation) the need to get Madame Isabella Lybbe up to sign her approval for the Allegation meant that this could only be done on the day of the marriage.  This still sounds a bit tight, though.  Further I would like to see a photo of the register itself just to make sure that it had not included the allegation date which was then mistaken for the marriage date by the Harleian transcriber.
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From FamilySearch:

groom's name:                     Philip Powys
bride's name:                     Isabella Lybbe
marriage date:                    19 Dec 1730
marriage place:                   Saint Benet Pauls Wharf,London,London,England
indexing project (batch) number:  M00136-1
system origin:                    England-ODM
source film number:               547508, 574439, 845242
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March 2021, TFPL: Transcript, by me, of the actual marriage register on Ancestry:

[ City of London  >  St Benet Paul’s Wharf  > 1728-1742 ]

“Marriages in 1730
“. . . .
“Powys and Lybbe
“Philip Powys Esq of Isleworth Midx B and           19 December
“Isabella Lybbe of Hardwick Oxon S by Mr
“Wooddison Cur of Goring Oxon
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