ONE PROBLEM with 10cc's first album was that it hit you hard on first listening, but often failed to hold up to repeated playings. Perhaps it was the feeling that 10cc was a contrived studio group; a gimmicky entity of pop pundits who conceived something new simply for newness' sake. But with Sheet Music, 10cc emerge as a mature unit, sprouting a real depth of scope and performance which their debut LP lacked. Now they use their varied talents not to create a tasty dessert treat, but more importantly to provide us with a satisfying main course.