BELGIUM REVISES DOWN 1987 GNP GROWTH FORECASTS The Belgian government has lowered its forecast for the nominal increase in gross national product in 1987 to 3.5 pct from an originally forecast 3.9 pct, Prime Minister Wilfried Martens said in a statement to parliament. He said this revision, which brings government forecasts more closely into line with those by private institutions, takes account of an anticipated slowdown in the world economy and international trade. But he said the impact on the Belgian economy will be limited due to recent agreements on wages and working conditions agreed by employers and unions. Martens did not say how much the government expected GNP to grow in volume terms. However, last month the government's Planning Bureau said it had revised its 1987 forecast for this to 0.9 pct from 2.0 pct. In 1986, Belgium's gross national product rose 2.15 pct in volume terms against a 5.9 pct nominal rise.