The letter was part of the ruse at the end of ‘Tweedledum’.
Knowing that any letters would have to go through the camp’s censor, a letter saying anything to the contrary regarding W/Cdr. Marsh’s lack of sanity continuing at home would jeopardise genuine cases for repatriation due of illness, in the future.This is backed up by a real life incident on which the episode is based, in which the escapee was indeed proven entirely sane and healthy after the war ended in 1945.
