The Nadezhda stopped September 28th, 1803 in Falmouth, Cornwall to obtain salt meat [Voyage Round the World in the Years 1803, 1804, 1805, & 1806, pp. 35]. Imperial ambassador to Japan Nikolai Rezanov took the opportunity to go to London [The First Russian Voyage around the World, pp. 16] and the ship was forced to wait for him; he returned on October 5th.
What did Rezanov (and, presumably, his suite of gentlemen) do in London in those first couple of days of October 1803?