Haydn was in London in 1791 when he performed in a concert led by Johann Salomon (a violinist/composer).
Salomon played the first violin and led the orchestra, and Haydn sat at the harpsichord, keeping the band together by an occasional chord or two, as the practice then was. Great composers have not always been great conductors, but Haydn had a winning way with his band, and generally succeeded in getting what he wanted. An interesting anecdote is told by Dies of his first experience with the Salomon Orchestra. The symphony began with three single notes, which the orchestra played much too loudly. Haydn called for less tone a second and a third time, and still was dissatisfied. He was growing impatient. At this point he overheard a German player whisper to a neighbour in his own language: “If the first three notes don’t please him, how shall we get through all the rest?” Thereupon, calling for the loan of a violin, he illustrated his meaning to such purpose that the band answered to his requirements in the first attempt. Haydn was naturally at a great disadvantage with an English orchestra by reason of his ignorance of the language. It may be true, as he said, that the language of music “is understood all over the world,” but one cannot talk to an orchestra in crotchets and semibreves.
Source: Cuthbert Hadden, J. (1902) Haydn. London: J. M. Dent, p. 81.