Ann the daughter of the couple above, died a spinster in Kensington, England.
Her father(Robert's) will mentions 6 children, Harriot, Catherine, William, Anne, Elizabeth and Mary. The girls all received substantial amounts of money and a yearly annuity. http://discovery.nationalarchives.gov.uk/details/r/D631502
The margins of the last page of the will contain later additions concerning the residuary estate . It's very tiny and I haven't yet managed to find the date but it did mention, Ann Pitt, spinster daughter deceased.
This lead me to the
||Will of Ann Pitt, Spinster of Kensington , Middlesex|
.http://discovery.nationalarchives.gov.uk/details/r/D435733 This will was not dated and not witnessed. It was probated in 1781 and may have been written some time before her death ( obviously some controversy since there is an affidavit from her banker confirming that it was in her handwriting)
In it Anne mentions her sister Katherine Needham and her brother the Earl of Chatham.
Because 1781 was after the death of the first Earl of Chatham (Pitt the Elder), I thought that this Ann could possibly be one of his children According to wiki, and the pedigree below. He did not have a daughter Anne
This pedigree confirms that Anne's sister married a Needham https://books.google.co.uk/books?id=JG2km_lggToC&pg=PR28-IA3&lpg=PR28-IA3&dq=Katherine+Needham+Pitt&source=bl&ots=SDoWjb-_SJ&sig=mdfECetev-P-NroFldfDao5nSo8&hl=en&sa=X&ved=0ahUKEwit7J237prOAhVJNhoKHcrkDGoQ6AEIPDAL#v=onepage&q=Katherine%20Needham%20Pitt&f=false
(I viewed both wills on ancestry.co.uk )