A Boeing 747 had just landed on runway 19, and was in the process of vacating the runway. At the same time, a King Air aircraft was given a clearance to cross the runway at taxiway "M". The pilot of a Beech 99 requested a take off clearance from the intersection of runway 19 and taxiway A4. The ADC (aerodrome controller) assessed that both the B747 and King Air would be clear of the runway in time and issued a take off clearance to the Beech 99. As that aircraft entered the runway, and commenced the take off run, the crew observed the King Air about to cross the runway. They rejected the take off and queried the take off clearance. The ADC saw no safety risk and recleared the aircraft for take off. The ADC was probably surprised to a minor extent about the rolling take-off, however he judged that there were no safety implications in allowing the take off to continue. The distance between taxiway A4 and taxiway "M" was 1410 metres.