In ATC back in the 80's at an en route centre. 2 aircraft converging , B747 and HP7 at nearly 90°angle at 6000 ft.Spotted just as phone rings to warn from adjacent unit. Avoiding action climb and descent given. The Herald crew were asked if they had seen the 747. "No, we were IMC,but we heard him"!Wasn't a good feeling for all involved!