Her baptism:

Name:                    Henrietta Maria Powys
Event Type:              Christening
Event Date:              8 Feb 1799
Event Place:             Leigh, Lancashire, England
Event Place (Original):  Leigh, Lancashire, England
Gender:                  Female
Father's Name:           Thomas Powys
Mother's Name:           Henrietta Maria

The image for the above was found on Ancestry and reads as follows:

>   Leigh, St Mary  >  1798-1812

“1799 Christenings at Leigh Church  (11)
“. . . .
“April
“. . . .
“28
“Henrietta Maria dr of the Honorable Thomas Powys
“and Henriette Maria his wife was baptized Febry. 8th &
“Received into the Church April 16th 1799”
“[Abode:]  Atherton”
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In 1881 a family of Colquhouns was in Chartwell, Kent with a daughter Lilian Powys Colquhoun; probably her descendants.

Chartwell????  The Prime Minister's residence???
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No death record on FreeBMD; Scotland?

But in fact in London though under Colguhoun:

Name:                          Age at Death (in years):  
COLGUHOUN, HENRIETTA MARIA     70  
GRO Reference: 1870  M Quarter in SAINT GEORGE HANOVER SQUARE  Volume 01A  Page 254
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Their children:

Archibald Campbell Colquhoun  19 Aug 1828  St Mary, Mortlake

No baptisms found in Scotland, nor any civil registrations.

In the 1841 census there is a young John in their house in Putney, aged 10 and born in Scotland.

No further census entries until 1921 when they must both have been dead.

None of Jphn the father, Henrietta, Archibald and John the Younger were seen in any further censuses bar 1921 for a John the Younger.  There were several John Erskine Colquhouns but I can’t identify him with these parents.

One FamilySearch tree thinks he was indeed the son of John and Henrietta and was born in 17 June 1831 at Stoughton Grange, which was a mansion of the Powyses.

Perhaps the problem is solved by the Gravestone for John Campbell and Henrietta Maria in the Holy Trinity churchyard, Sevenoaks, Kent as on the third side of this is a memorial for John Erskine and his wife, clarifying that he was of that family.
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Their marriage from the image of the regisgter page, thanks FamilySearch and Ancestry, transcript by me:

“                           [ Page 214 ]
“Marriages solemnized in the parish of Warrington
“  in the County of Lancaster in the Year 1827
“. . . .
“John Campbell Colquhoun of the Parish
“of Kilpatrick in the county of Dumbarton North Britain
“and Henrietta Maria Powys of this Parish
“were married in this Church by licence with Consent of
Parties this tenth Day of
“September in the Year On Thousand eight hundred and twenty seven
“   By me James J Hornby  Officiating Minister
“This Marriage was solemnized between us: John Campbell Colquhoun, Henriette Maria Powys
“In the Presence of Horace Powys, Eleanor Powys
“No 64

I am not sure who Horace and Eleanor Powys were.  Eleanor might have been Henrietta M’s immediately younger sister.  Horace could have been her young brother Horatio, aged 22 or so, later the bishop of Sodor and Man.
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