Martin P-L writes:

1734 born.  His godfathers were Sir Wm. Twysden (his great-uncle, 5th bart) and Sir Philip Meadows jnr. [TFPL: and his godmother was Mrs Lybbe, his grandmother]
1753 matriculated at St John's Oxon.
1759 admitted to Inner Temple on 22 Jun
1761 Bachelor of Civil Law
1762 married Caroline Girle, whom he had met at a party in Berks.
1764 Their first child was born, but died after 9 months.  Boys were born in 1765 and '8, and a girl in 1775.
1778 made Deputy Lieutenant of Oxon.
1784 or 86  finding Hardwick too big after the departure of their two sons, they let it to a Mr Garth or Gardiner, and moved with their daughter to Fawley rectory nearby, where their brother Thomas was installed..

Philip seems to have been a quiet, unassuming person, to whom Caroline was devoted (Diaries p.298).  In appearance he was tall and thin.  Smallpox had left him with a good, rough, manly face (Diaries).  He was weak in health, but frequent visits to Bath extended his life to 74 years.  There is (or was) a miniature of him.

1809 returning from Quarter Sessions at Oxford, he fell into a pond at Fawley and drowned (says Ms Potter, an American lady who at one time was working on a new edition of the Diaries.  Ursula met her; I have not.  She was, I think unable to interest a publisher, so abandoned the project.  Ursula gave me the 60 pages of outline and samples that she had submitted to publishers.)  Unusually in the family, he had made no will.

Whitchurch Burial Register:
  "1809 Apr 20  LYBBE POWYS   Philip, Esq of Fawley, Bucks, 75, accidentally drowned"
This confirms part of Mrs Potter's account.

Gentleman's Magazine For the Year 1809, Volume LXXIX, Obitiuaries p. 479:
In his 75th year, Philip Lybbe Powys, esq. of Hardwick-house, co. Oxford, in the commission of the peace and deputy lieutenant of that country, and brother to the Dean of Canterbury.  He had attended his duty, as a magistrate, at the Quarter Sessions on the preceding day; and had returned from Oxford this evening with another gentleman, in a postchaise, as far as Assenden, where he got out to walk to his residence at Fawley; but, from the darkness of the night, missed his way, fell into a pond, and was drowned.

(Was he the worse for wine, one wonders?)
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In her diary for 6 Nov 1802, Caroline Powys wrote: my Dear Phils Birth day, 68 Born 6 Nov New Style 1734.
This has the correct eleven day difference.
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In 1805, he, or his son, was appointed oe of many Commissioners for Berkshire for raising a tax on pensions, officees and personal estates and a land tax.  (Possibly something to do with the new-fangled income tax?)
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His Inner Temple record:

Powys, Philip Lybbe

Type of entity                         Individual person
Parallel forms of name                 Powis, Philip Lybbe
Father                                 Powys, Philip
Father's occupation                    esquire
Father's address: town                 Hardwick
Father's address: county               Oxfordshire
Father's address: country              England
Relationship                           son and heir apparent
Functions, occupations and activities  esquire
Admission year                         1759
Admission date                         22-06-1759
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Whitchurch Marriage Register:
  "1762 Aug 5   POWYS   Philip Lybbe of Hardwick in this parish
                         GIRLE     Caroline of Caversham
                    Wit: John WHISTLER, Philip POWYS                    Lic."

In her diary for 5 Aug 1802, Caroline Powys wrote: our Wedding day married in 1762.