HONGKONG'S business community have put $18 million into local sport in the past 15 months - but more is needed to maintain the momentum at all levels of sport. That was the message from the chairman of the Hongkong Sports Development Board (SDB), David Gledhill, at a luncheon for present and potential sponsors yesterday. Guests heard that the SDB's sports sponsorship advisory service had raised $18 million. Gledhill said: ''I think it is very important for the corporate sector to support sport, especially now the Government have indicated there will be no increase in the amount they give to the SDB, other than inflation-linked. ''This means we will have to rely on money from the Royal Hongkong Jockey Club and from the corporate sector.'' The SDB, a statutory body set up on April 1, 1990, to promote and develop sport and physical recreation in Hongkong, received $55 million from the Government for the 1992-93 financial year and can expect to receive around $60.5 million for the next year. ''During its first 15 months, the sports sponsorship advisory service raised $18 million, which is equal to 25 per cent of the budget from the Government,'' added Gledhill. ''That is very impressive - and we thank them for their generosity and hope they have received benefit.'' Gledhill described the Jockey Club as ''the corporate fairy godmother of them all'' for their donation of $250 million last year. With the SDB and the Hongkong Sports Institute - the Sha Tin centre for the territory's elite athletes - set to merge on April 1, Gledhill is hopeful the Government will provide money for the Institute, whose Jockey Club resources are running out well ahead of schedule. The SDB are hoping to raise $9 million for the current financial year and have so far brought in $7 million.