Hi Bob,
I've just come across a Lancelot Armstrong who married Isabell Rewcastle in 1798 at Corbridge (next to Hexham, Northumberland.)
In the baptisms for Corbridge, i've found John Armstrong, aug 7th 1799 son of Lancelott Armstrong of Corbridge, Surgeon, native of Langholm, Scotland and Isabell Rewcastle of Corbridge.
My 5th great grandparents were John Armstrong and Ann Rewcastle, they were married at Hexham, Northumberland in 1768.
As far as i can tell so far, Ann Rewcastle and Isabell Rewcastle were from the same family, cousins or aunt and neice.
My 5th great grandfather John Armstrong had a relative Dr John Armstrong (1749-1815) whose father was John Armstrong, (1709-1779) Apothercay and Surgeon of Brampton, Cumberland.
Dr John Armstrong (1749-1815) had relatives Charles and William Armstrong who were East indies merchants, later on they went to Mobile, Alabama where they were cotton merchants before returning to England.. I understand they may have had family out in the USA.
Charles and William Armstrong father, another John Armstrong had Armstrong relatives in Graitney, Kirtleton and Bowholm, Canonbie.
Does this connect with anything you have?
Regards
Rewcastle.
p.s. found this site last night, just search for Armstrong.
link