Jorge.

You are welcome.

Both of these works identify Lieutenant Colonel John Grant as the one taken prisoner and freed by the Spanish.

The first respectable spy: the life and times of Colquhoun Grant, Wellington's Head of Intelligence. Jock Haswell. Hamilton 1969.

The Intelligencers: the story of British military intelligence up to 1914. B. A. H. Parritt. Intelligence Corps Association, 1983.

Good hunting.

Ron