CHINA Motor Bus (CMB) is battling to prevent elderly passengers shunning its cross-harbour services when Kowloon Motor Bus (KMB) begins offering fare concessions next Monday. KMB will charge passengers aged over 65 on its 50 cross-harbour routes $2.70 a trip upon the production of their identity card while CMB will charge $5.30 for the same service with the Executive Council's approval of the new fares. A CMB official said yesterday the company was seeking clarification of Financial Secretary Mr Hamish Macleod's offer of incentives to encourage the introduction of similar concessions by CMB. Mr Macleod said in his Budget speech the Government would waive licensing fees for buses to encourage an early introduction of concessions. ''The value of the concessions seems to make it difficult to contribute to a financially attractive package ,'' the CMB official said. CMB was prepared, in principle, to offer half-fare concessions to elderly passengers on most island routes, except cross-harbour services, he said. Transport Commissioner Mr Rafael Hui Si-yan said yesterday providing concessions for elderly passengers would cost CMB an estimated $20 million in annual revenue and the incentives would restore about $10 million of that figure.