Warren, 
I can't find much to help you except a possible lead on the Horner.  In 
Memoirs of Francis Horner, Edinburgh, 1849, there is a reference to your 
letter on p. 244, but alas, not the content you wanted.  Interestingly in 
the light of the film, Madness of King George (good characterization of Pitt 
and Fox), Horner says: "I am inclined to think that the king, if he does not 
die from bodily weakness, will recover his present madness;...I am all for 
the monarchy, ..." 
Good luck, Bill Campbell