DESPITE the political uncertainty surrounding Hongkong, and the possible effects of the slowing of the Chinese economy, a solid majority of Japanese companies in the territory intends to maintain or increase investment here. A survey carried out for the Japanese-language Hongkong Business Post newspaper indicated that 38 per cent of locally based Japanese companies were preparing to increase their direct investment here over the next one to two years. A further 55 per cent of the respondents were content to maintain their current level of investment, while only one per cent said they were considering reducing their exposure. A hefty majority also backed investment into China. Of the 181 companies which replied to the poll - 900 were asked - 39 per cent had already invested, while a further 29 per cent planned to. But, weakening the theory that Hongkong is seen mainly as a stepping stone to the mainland by Japanese companies, a significant 27 per cent said they had no plans to invest there. The main misgivings about investment in China were the lack of infrastructure, which worried 46 per cent, and political risk, a concern to 30 per cent. Governor Chris Patten can count on very little support for his reforms among Japanese businessmen: only two per cent believed they would benefit the Hongkong economy. Forty six per cent said reforms that aggravated China would be counter-productive, and 27 per cent expressed concern that retaliatory action by the Chinese would damage the Hongkong economy. With or without political problems, the Japanese in Hongkong are sceptical about the ''one country, two systems'' concept being a total success. According to 74 per cent, the idea is only partially feasible, while only 17 per cent expected it to be implemented totally. Nevertheless, only two per cent gave it no chance of success. Most Japanese companies link Hongkong's future prosperity to that of China, with 51 per cent replying that one depended on the other. Twenty per cent expected the current prosperity to continue indefinitely, while 16 per cent expected it to continue until 1997. Expectations of the airport project being complete by the June 1997 are relatively low among Japanese companies here, although 60 per cent believe it will bring with it a more vital construction industry. Only 19 per cent expected the original deadline to be met, while 24 per cent thought it would be delayed until the end of 1997, and 35 per cent believed it would take until the end of 1998. The companies canvassed for the survey were firmly bullish about the prospects for the stock market in Hongkong. More than 30 per cent expected the Han Seng Index to close the year at 8000, while 27 per cent expected it to reach 7500, compared to last night's close of 6858.08. There were a small group of super-bulls prepared to pencil in 9000.