On the 1851 census, Mary gives her birthplace as Bristol (and birth year as 1777). The 1841 census shows Mary and Lewis residing in Swansea, but Mary is recorded as "not born in county".
Do you have the entry relating to Mary from the National Probate calendar, or Lewis's entry in the Dictionary of National Biography - both have a "Llewelyn" connection?
Family Search has a Mary ADAMS, daughter of Sarah ADAMS baptised in 1770 in Weare, Somerset. Weare is approx. 15 miles SW of Bristol. Although the date is a tad early, I note from the 1851 census that Mary is a year older than her husband - might she have given a later birth year to minimise an even greater age difference?
Pennie