Hi Chris,
French himself makes no specific mention in his '1914', and Vol. 1 of the OH is similarly unrevealing, with the reproduction of Op Order No.23 issued on Sept. 12th merely recording that it came from General headquarters, but giving no location.
E. Swinton ('Eyewitness') records his stay billeted in the house of an old lady, and an office in an unfirnished room at the Chateau gatehouse, whilst French and his staff were quartered in the Chateau itself. But although he speaks of G.H.Q he also does not record it's specific location.
There is further mention of French's accomodations on
http://www.hellfirec...ir/charles8.htm but it doesn't entirely accord with Swinton and still makes no specific reference to GHQ's location.
Can't find anything as specific as you would like to see, sorry. One could understand contemporary accounts concealing locations for reasons of OpSec. But not accounts written many years later on.
Simon.