I found this in an article entitled Musical Intelligence in the Whitehall Evening Post, 17 July 1783:
Hayden [sic], next autumn, comes to London. Phlegm, and, in all pecuniary concerns, extreme caution, are among the leading characteristics of this great composer; insomuch so, that last winter he could not, without uncommon assurances, be prevailed on to send his new music over to Lord Abingdon's Grand Concert. Graaf, with Weiss and others, are gone to Germany and Switzerland for the summer; to return, however, to the Subscription Concert in Hanover-Square.
The "Weiss" it mentions is Jean Gaspard Weiss. Weiss returned permanently to Mulhouse in 1783, and considering the final subscription concert that I can find in 1783 was actually on May 21 (which, by my reckoning could be considered around the start of summer) I'm not sure that he actually did ever return to London for any more concerts that year. He certainly did not perform in any subscription concerts in London after 1783.
And I still have no idea why they were talking about Haydn's phlegm.
Update: Apparently "phlegm" can mean "calmness of temperament" as well as the mucus that comes out of the back of your throat. Who knew?!