[Phoenix]

That is brilliant! Thanks, Kite.
I did see that Mary gave Sussex as her birthplace on the 1851 census, but it was not clear from the transcript whether that was original information, or added.

I need access to "In search of the Forlorn Hope", which lists where army units were at any given time. However, since it is selling on Amazon for nearly £100, I'll curb my enthusiasm.

Quoting from Wikipedia:

"In 1807, the regiment was deployed to Jersey in the Channel Islands for garrison duty, remaining there until 1808, when it was deployed to Spain to take part in the Peninsular War.[5] The regiment took part in the Battle of Corunna in January 1809 and was evacuated from the Peninsula later that month.[5] The regiment took part in the disastrous Walcheren Campaign in autumn 1809 and, having reverted to the title of 76th Regiment of Foot in 1812,[1] returned to the Peninsula in 1813 seeing action at the Battle of Nivelle in November 1813[9] and the Battle of the Nive in December 1813.[10] It then embarked for North America for service in the War of 1812 and saw action at the Battle of Plattsburgh in September 1814.[11]
The Victorian era

The regiment did not return from North America until 1827.[5]"

That sorts out Thomas (apparently most of the recruits came from Nottinghamshire) but how on earth did Mary get out to Canada?