THE much-loved, truly sad legend of the ''butterfly lovers'' - theme of Chinese opera, theatre and story-books - is now a sumptuous movie, starring the much-loved showbiz darlings, Nicky Wu Chi-lung and Charlie Young Choi-nei. To coincide with the screen version of the story, Ultimate 903 of Commercial Radio II presented ''The Lovers - Nicky Wu and Charlie Young In Concert'', held last week at Queen Elizabeth Stadium. The show re-told the legend in song, against a dazzling stage set that combined ancient and modern times. Wu and Young delighted a highly responsive audience with their music and costume changes that wove in and out of classical Chinese and contemporary styles. The stars sang their latest hits and, of course, the theme song from their movie, The Lovers. The guest performers were the entertaining Soft and Hardcore Kids.