Martin P-L writes:

1743     born
1755-9   at Eton.
1760     admitted as fellow-commoner to King's Cambridge.
1768-9   High Sheriff of Northants
1770     sold Henley Hall to Ralph Knight, who reconstructed the house.
1772     married Eleanor Mann, dau. of Galfridus Mann of Kent.
1774-96  MP for Northants.  He was an effective and frequent speaker in the Commons.  Independent and strong minded, he opposed the war against America and, though twice offered a ministry, he preferred to keep his independence.  More than anyone, he supported Rockingham.  Though he was generally in favour of Pitt, he thought he should have resigned after being defeated in the Commons over the India Bill, and from then on he opposed him.  Eventually, however, he joined the government side and received a peerage for his long and distinguished service.
1800 died.

There is a long account of his parliamentary career in the official History of Parl., by Nannier & Brooke.
His integrity is attested by Sir N. Wraxall in his Memoirs, vol ii, p.217.

He and Eleanor had five sons, six daughters.  The fifth son, the Hon. Henry Powys, rose to Captain in the 83rd Regiment of Foot, served under Wellington in the Peninsular, and was mentioned in his despatches.
He was wounded in the attack of Picuzina and died of wounds at Badajoz in 1812.
Another son, Charles, was in the Royal Navy and died of yellow fever at Jamaica in 1804.
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The transcription (by me) of his baptism from an image of the Register page is:

                 [ St George, Hanover Square  >  1738-1751 ]

“                            Baptisms
“                              1743
“.  .  .  .
“Baptiz’d                       June                          Born
“.  .  .  .
“2.     Thomas, Son of Thomas Esqr & Henrietta Powis          4 [May]”

Note that I have deduced from the Register that he was born in May, not Mar.  Just checked with Complete Peerage and it agrees with me.  Note also the spelling of the surname.
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Jan 2021, TFPL: I am still unable to give their children into their birth orders by sex as I have yet to find all the baptisms.  But I have confirmed or added half a dozen today.  Five more to go.

There is a bare single entry on each of FindMyPast and Family Search while Ancestry has both those and all the rest that I have found, with images of the baptismal register.

Sept 2021, TFPL: I have now found all of fourteen baptismal records.  This is two more children than in Oswald Barron’s genealogy of the family but he omitted the two early children who died young: Louisa and Horatio.  I have also sorted out that there were two Louisas, the second having the additional forename of Horatia so that she was named after both her pre-deceased siblings.

Only the first daughter married and had ten children.  The last daughter was the only other of the eight daughters to marry but she did not do so until she was 58 and thus well beyond child-bearing.

Only the first and third of the six sons had children, the first having another large family, of twelve children and the third, Littleton, having a comparatively modest six.

So Thomas and Eleanor had 14 children and 28 grandchildren.
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From the Northants FHS booklet of Mon Inscriptions for St John the Baptist, Thorpe Achurch, pub 2015:

“Near this spot lie interred the perishable remains of The Right Honorable THOMAS, Baron Lilford.  He was chosen to represent the County of Northampton in four successive Parliaments; He was called to the House of Peers A.D. 1797.  And on the 26th day of January A.D. 1800 in the 57th year of his age he sunk (alas how soon!) under the unwearied exercise of public and domestic virtues.  In the former how conspicuous he shone.  The unanimous approbation of his electors, during a period of more that 20 years.  The honors of His Sovereign, And the admiration of his Country, abundantly declare in what manner he fulfilled the latter.  Is recorded in the bosom and commemorated by the sorrow of his family.  His affection was tenderness, His authority was friendship, Let him not have lived in vain!  He hath not bequeathed to his posterity.  The fruitless gift of unavailing regret: H hath bequeather to them and to mankind, A bright example to be followed of piety, benevolence and wisdom.”
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Their marriage transcription by FindMyPast:

First name(s)           Eleanor
Last name               Mann
Residence               Cottesmore
Marriage year           1772
Marriage date           31 Mar 1772
Spouse's first name(s)  Thomas
Spouse's last name      Powys
Spouse's residence      Lilford, Northampton
Denomination            Anglican
Place                   Cottesmore
County                  Rutland
Country                 England
Archive                 Record Office for Leicestershire, Leicester & Rutland
Archive reference       DE1920/4
Record set              Rutland Marriages
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