AN association controlling more than half of Hong Kong's urban taxis will today seek an eight per cent increase in fares. The increase, proposed by the Taxi Operators' Association, is two to three per cent less than those proposed by other taxi associations. ''We prefer a gradual increase in flag-fall charges every year so that the public will accept the adjustment,'' said chairman Leung Shiu-cheong. He said the increase was to cover the rising cost of parts imported from Japan and inflation. A Transport Department spokesman said a decision would be made by January.