British carrier pigeon, named Cher Ami, delivered life-saving message to troops
Battalion of 700 men was cut off from troops in France during First World War
Unaware of their position, Allied troops began bombing area killing hundreds
Hero carrier pigeon flew 25 miles to deliver location despite being badly injured
Although 194 men were saved after the location was learned, 150 were captured
Now rare account from PoW soldier revealed in archived documents and letters