Dette,
Following on from Annette & her great find

From yourself Reply #27
“All of Mark's family are dark skinned - a lot of the time they are
mistaken for native Indians whereas they are actually part British, Maruitian and Portageuse”
From Annette Reply #29
“James Glenday, bachelor, Gunner in the 3rd. Battalion Artillery and Frances Oliver of St. Thomas Mount,
Indo Briton”
Google says “Definition of Indo–Briton, a person born in India of mixed Indian and British descent”
Could this mean he was born in India too, to an Indian mother (confusing)?
As Annette has put you on the right trail & with her suggestion “You need to locate and get a check done of Muster Rolls from 1841 to 1848 in Madras. This should then give you the details of where he came from and his age”.
Hopefully that will uncover some good info. to further your search?
Could this be the birth & bapt. for Frances married to John/James who’s marriage took place in Madras?
I thought the names to be too much of a coincidence?
https://familysearch.org/ark:/61903/1:1:FG4Z-HFDFrances Oliver
Born 18 Jan 1822
Christened 25 Nov 1822 at Arcot, Madras, India
Father William Charles Oliver
Mother Mary Frances
Annie