The music of the 18th century is generally one of the weakest areas in the output of the National Symphony Orchestra. This is rather more than a pity, since the 18th century includes all of Mozart and most of Haydn. Is Gerhard Markson (right) the new principal conductor elect of the National Symphony Orchestra, going to be the man to turn this situation around? Find out, at least as far as Haydn is concerned, at the National Concert Hall over the next two weeks. Markson is conducting Haydn's Symphony No 31 (the Horn Signal) along with Korngold and Mussorgsky/Ravel next Wednesday at 6.30 p.m. And Haydn's Symphony No 102 in B flat partners Milhaud's Le boeuf sur le Toit in next week's lunchtime concert, on Friday 21st, also at the NCH.