Her baptism from Family Search (but without baptism date...):

Name Barbara Slaney
Gender Female
Birth Date 22 May 1717
Birthplace ST KATHERINE CREECHURCH,LONDON,LONDON,ENGLAND
Father's Name John Slaney
Mother's Name Barbara
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The burial register, quoted by Emily J Climenson:

"On January 14th, 1801, Barbara, widow of John Girle, aged 86, from Henley, Oxon"

She was godmother to her great-grandson, Henry Philip Powys, on 2 Jan 1792.

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On p. 370 of the diaries, Mrs Climenson writes:

“Mrs Girle, Mrs Powys’ mother, had a half-sister, who married first a Mr Phelp, by whom she had a daughter, married to the Marquis de la Piere.  AFter the death of Mr Phelp, Mrs Phelp remarried a Mr Floyer.  A daughter [Elizabeth] by this second marriage married Pennystone Powney of Ives Place, Berks on December 20 1776.  This lady writes to following [to Caroline Powys] and sends it by her son …”

The question is what was the relationship of Mrs Phelp/Powney to Barbara Slaney who married John Girle?  There is no other evidence that I can find.

Jan 2013, TFPL: The above report allows the deduction that there was an Elizabeth Floyer born sometime between, say, 1740 and, say 1760 and who would thus have been aged between 16 and 36 at marriage on 20 Dec 1776.  This produced these birth entries on FamilySearch:

name: Elizabeth Floyer
gender: Female
baptism/christening date: 09 Jun 1754
baptism/christening place: ST MARY STAINING,LONDON,LONDON,ENGLAND
father's name: Peter Floyer
mother's name: Dorothy
indexing project (batch) number: C02247-1
system origin: England-ODM
source film number: 374494

and

name: Elizabeth Floyer
gender: Female
baptism/christening date: 12 Sep 1748
baptism/christening place: SHINFIELD,BERKSHIRE,ENGLAND
father's name: Peter Floyer
mother's name: Henrietta
indexing project (batch) number: C01849-2
system origin: England-ODM
source film number: 88393

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The Society of Genealogists has a Floyer marriage to Powney on the index of Vicar General Marriage Licence Allegations:

Licence date   19 Dec 1776    
Year         1776    
Bride Surname   FLOYER    
Groom Surname   POWNEY

I have ordered a copy of the full thing and this gives:

“19 Dec 1796
On which day appeared Penyston Portlock Powney of Maidenhead in the county of Berks Esqre and made Oath that he is a widowr and intends to marry with Elizabeth Floyer who hath had her usual place of abode in the Parish of St Alban Woodstreet London for upwards of four week last past that She is a Spinster of the Age of twenty one years and upwards.

“Not knowing or Believing any Impediment by reason of any Precontract Consanguinity Affinity or any other cause whatsoever to hinder the said intended Marriage And he prated a Licence for the to be married in the Parish Church of St Alban Woodstreet aforesaid.

“[Signed]  Pen: Port: Powney”

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The Society also had this entry from Vicar General Marriage Licence Allegations index 1694-1850

Licence date   12 Jul 1753
Year         1753    
Bride Surname   PHELP    
Groom Surname   FLOYER

The full record is:

“12 July   [1753]
On which day appeared personally Peter Floyer of the parish of St Mary Staining London aged thirty years and upwards and Bachelor and alleges that he intends to marry with Dorothy Phelp of the Parish of St Andrew Holborn in Middx widow.

“Not knowing or believing any impediment by reason of any Precontract Consanguity Affinity or any other lawful me ands whatsoever to hinder the said intended Marriage. Of the truth of which he made Oath and prayed Licence for them to be married in the parish Church of St Anne Blackfriats or St Mary Aldermanbury London.

“[signed] Peter Floyer”

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The following from the A2A database gives some information of the first marriage of Barbara Slaney’s half sister, defining that her first name was ‘Dorothy’ as was that of her daughter by her first marriage, the daughter being born Dorothy Phelp:

Deed of Covenants  ACC/638/111  21 Feb. 1824

Contents:
1. Rev. Frederick William Blomberg, Kirby Hall, York, D.D. Canon of St. Paul's and Peter Styan, Baker Street, Esq.
2. Dorothy Marchioness De la Pierre, Hampton Wick, widow
3. Stephen Thomas Cole and George Francis, Twickenham, Brewers
4. Francis Burton, Upper Brook Street
Whereas: Indenture 25th March 1780. Parties: 1. Charles Joseph Vibert, marguis de La Pierre, son and heir of Joseph Vibert, Count de Massinghy, 2. Joseph Vibert, Count de Massinghy, 3. Dorothy Marchioness de La Pierre, then Dorothy Phalp, 4. Daniel McNamara and Elborough Woodcock esq.
Marriage settlement for Dorothy Phelp and Charles Joseph Vibert.
Dorothy Phelp seized of various copyhold messuages in manors of Hampton Court, Ealing, otherwise Zealing with Acton and Richmond otherwise West Sheen, in reversion, expectant on death of mother Dorothy Floyer.
Covenant to surrender before marriage to several Lords of the manor, to the use of Daniel Macnamara upon trust to invest money arising from sale of premises for trusts.
Whereas: death of Elborough Woodcock November 1794, and death of Dorothy Floyer 1798. By deed poll by Charles Joseph Vibert and Dorothy his wife 11th October 1799 Frederick William Blomberg appointed as trustee in place of Daniel McNamara.
By indentures of lease and release 15th and 16th September 1800 between 1. Charles Joseph Vibert Marquis de la Pierre and Dorothy his wife, 2. Frederick William Blomberg, 3. Lawrence Nichell D.M., 4. John Pearson. gent.
Lawrence Nichell appointed trustee for Daniel McNamara and Frederick Blomberg for Elborough Woodcock.
Indenture 20th February 1824 between
1. Dorothy Marchioness de la Pierre,
2. Frederick William Blomberg,
3. Peter Styar..
Reciting that at Court Baron for Ealing Manor 15 April 1805 Lucinda McNamara, wife of Francis, Ardelomy, County Clare, Mary Bouchier, wife of John of Kelfruish Limerick, Margaret O'Brien, Harley Street, Marlebone co-heiresses of Daniel McNamara, admitted tenants of copyhold messuages in Ealing manor. (See Acc. 638/109).
Reciting death of Lucinda MacNamara, leaving Maria Faulkner, daughter, as heir; death of John Bourchier and Lawrence Nichell March 1823, and death of Charles Vibert 31st March 1814; copyhold premises etc. vested in Maria Faulkner, Mary Bouchier and Margaret O'Brien.
Frederick William Blonberg and Peter Styan at wish of Dorothy Marchioness de la Pierre, widow, appointed trustees in place of Daniel McNamara and Elborough Woodcock.
Whereas: Frederick Blenberg and Peter Styan have agreed to sell premises to Stephen Cole and George Francis for £1,729.
Premises: Customary messuage, with yard, barn, etc. called Queen's Head and 3 acres of land, and land adjoining river Thames, messuage called Fox and Hounds with messuage adjoining and parcel of garden ground between Highway and Thames. To the use of Francis Burton, upon trusts. Schedule on last membrane.
Sealed and signed by F. W. William, Peter Styan and Dorothy de la Pierre.
Brentford, Ealing Manor.

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A further A2A document established that Dorothy Phelp, later Marchiones de la Pierre, had a daughter Mary Clementina Vibert:

Lease  ACC/638/110  26 July 1808

Contents:
1. Charles Joseph Vibert Marquis De la Pierre, Baker Street, Portman Square, Dorothy Marchioness, wife, Mary Clementina, daughter, 21 years.
2. Henry Cole, Daniel White and William Hames of Twickenham, Brewers
Premises: Fox and Hounds, Old Brentford.
Term: 21 years from 29th September 1807.
Rent: £30 p.a. - now doubtful whether lessors able to grant lease.
Therefore articles of agreement:- Consideration £150 paid by lessees, to be expended on repairs of premises.
Lease of messuage formerly called 'The Red Lyon', now The Fox and Hounds, with carthouse, garden etc., except walled gardens, and tenement with barn or blacksmith's shop belonging to the Fox and Hounds.
Lessors to be permitted right of way with horses, waggons, carts etc. through yard to Turnpike Road in front of the premises.
Covenants for payment of rent, that lessees will pay all rates, land tax etc. assessed on premises, to keep premises in repair and to leave premises in good state of repair at end of term.
Signed and sealed by 6 parties concerned.
Brentford

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There is this marriage record in the Register of St George, Hanover Square, Vol I for 1725 to 1787, Pub Harleian Soc 1886, p. 311, for 27 May 1780: “Charles Vibert De la Pierre, of Lincoln's Inn Fields, & Dorothy Phelp, of this parish. Licence**
** Witnesses, Dorothy Floyer and Joseph Vibert.”

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Note that I still have not found the maiden name of Mrs Dorothy Phelp who married again to become Mrs Dorothy Floyer.
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No record has been found for this marriage, as at December 2016.

But there are lots of records, wills particularly, to show that their daughter Caroline was the daughter of Dr John and Barbara Girle.
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