MANY NPC delegates are spending as much time on their own business than on deliberating matters of state in China's parliament. A report by the semi-official China News Service said among the delegates from Guangdong province, there were 12 mayors who made the best use of their time in the capital in ''horizontal liaison'' with central-government ministries. These mayors usually skipped lunches and dinners with the other delegates in pursuit of their own agendas. The mayor of the city of Yangjiang, Wen Paotian, carried with him to Beijing several proposals for an airport, railway and ports. He indicated after a few days of effort, he had secured investment for the new airport. Another busy mayor was the head of Huizhou, Li Jinwei, whose city had suffered badly from the retrenchment programme last year. Mr Li told journalists that to secure Beijing's support for his city, ''it is necessary to make many reports'' to top-level ministers.