A Crowded House . . . and Stage : Music review: The Academy of St. Martin in the Fields looked more like a symphonic ensemble than a chamber group at a sold-out UK/LA Festival concert at Cerritos center.

When is a chamber orchestra not a chamber orchestra? One easy answer: when it has more than 65 members. At that point, it becomes just another, standard symphonic ensemble, ready to play Tchaikovsky, or Sibelius, or maybe even Mahler.

Now numbering 71, England’s 35-year-old Academy of St. Martin in the Fields stopped Sunday night at the Cerritos Center for the Performing Arts, after a two-week tour of Japan, to open the classical music portion of the center’s third season as part of the UK/LA Festival. The orchestra is now on its way home via San Francisco and New York City.

Even swollen to its current membership, the Academy, under its founder-conductor, Neville Marriner, retains some of the polish and accomplishment of its more slender days. On this occasion it played familiar repertory, and with some finesse. Being an all-purpose orchestra may be a questionable goal, but it has its uses.

Among them is the showcasing of young and emerging talent. Sunday, Marriner and his ensemble introduced to Southern California one of its own--a 16-year-old violinist who began her training here at age 3.

Leila Josefowicz now lives in Philadelphia, where she is enrolled at the Curtis Institute, studying under Jascha Brodsky and Jaime Laredo. From her articulate and unfazed performance of the Violin Concerto by Tchaikovsky with Marriner and the Academy, one can without hesitation report her splendid promise.

Native musicality and physical poise are the starting points of that promise; rock-solid technique and communication skills form its core. The true conviction of the highly gifted adds that extra confidence one expects in a prodigious teen-ager.

Josefowicz’s playing of the concerto had depths and facets as well as stunning achievement; it could also boast a beautiful tone, a sense of continuity and good humor, especially in her coltish but controlled romp through the finale. The fates willing, Josefowicz is going to be an important member of the post-Midori generation. Sunday, she was not hurt by playing the 1708 “Ruby” Stradivarius on loan to her from the Stradivari Society of Chicago.

Marriner and his band contributed strongly to this debut--scheduled for a repeat in Carnegie Hall at the end of the week--despite the alarming brightness of the Cerritos Center’s acidulous acoustics.

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The first half of this sold-out event--1,600 listeners seated in the center’s “concert configuration”--also stressed loudness over subtlety, though the Academy’s players made valiant attempts at dynamic variance. And some genuinely quiet moments, particularly in the more tender portions of Elgar’s “Enigma” Variations, materialized briefly.

One could hear, then, that soft playing is certainly possible in this room. What is not possible, it seems, is any strong differentiation between a little loudness and a great deal of it--the layering of contrasting levels.

Marriner’s conducting of the “Enigma” set followed a pedestrian course; the work’s familiar contours emerged unimpeded, if usually unmotivated. In spite of interested playing from a few individuals--especially, principal viola Robert Smissen--more often evident were indications of musical feeling rather than feelings themselves.

A careful and virtually immaculate run-through of the Overture to Mozart’s “Zauberflote” began the program. One missed only the kinds of textural and instrumental transparency the 70-year-old conductor used to achieve when he led orchestras with fewer than four dozen members.