The English writer and director Mike Leigh is best known in this country for his recent films Naked, Life Is Sweet and High Hopes. For 25 years, however, he has developed an extensive body of work .for stage and film, all of it created from his own method of structured improvisation. His 1979 play Ecstasy, which is being produced Off Broadway at the John Houseman Studio Theater by the New Group through May 14, under the direction of Scott Elliott, marks Leigh’s first U.S. production.…