During her 2 1/2-hour show at the Kennedy Center Concert Hall Saturday night, British singer Cleo Laine proved herself a superb band singer. Her 4 1/2-octave voice was a flexible jazz instrument, wringing the rhythm from each word while maintaining crisp control of diction. Laine turned even a simple ballad like "Never Let Me Go" into a vocal showcase, shading the song with tropical colors, now imitating an earthy sax solo, now gliding on glassy high notes.

Laine's quintet, led by her husband, John Dankworth, provided tuneful but tame cocktail jazz-fusion settings for her vocals. Dankworth, who introduced himself as "the president of CLA--Cleo Laine Accoutrements," exhibited a pleasant liquid sax sound, which Laine often mimicked vocally. Of several extended instrumental excursions, the most expressive was "Belleville Blue," which featured vibe-like electronic piano noodlings and feathery flute voicings. Laine seemed a bit too comfortable in the soft musical upholstery, never challenged by the MOR meanderings of the material.

The set's high point was a quartet of poems set to music by Dankworth. Laine sailed sassily through W.H. Auden's whimsical poem "Tell Me the Truth About Love," which seemed a natural as a jazz song. Laine also showcased her stage training, delivering Noel Coward's "Mad About the Boy" as three distinct characters.