The plan was to fly from Amsterdam to London Gatwick, with only the crew on board, to position for a series of non-schedule flights the following day. The aircraft had a history of technical problems with the brake system and, on a previous sector that evening, there had been a build-up of pressure in the Yellow brake system; this manifested itself as a pressure indication on the right side of the dual brake pressure indicator and a hardening of the feel of the right brake pedal. The commander considered the commercial and technical ramifications of declaring the aircraft unserviceable at Amsterdam and decided, as no passengers were involved, to continue with the positioning flight to Gatwick. Contrary to the requirements of the minimum equipment list, he elected to take off with the Yellow hydraulic system depressurised and briefed the crew, at some length, on the consequences of this; included in the briefing was the need to increase required runway distances with inoperative ground spoilers and the requirement for the flight engineer to be ready to reinstate the Yellow hydraulic system immediately should it be needed for alternate braking. The aircraft left Amsterdam at 0005 hrs with the first officer as the handling pilot. The flight was uneventful and the aircraft landed at Gatwick at 0110 hrs; the first officer noted that the right brake pedal was difficult to depress and, using the tiller, it seemed to require more effort to turn the aircraft to the right than to the left. The consequences of parking with the Yellow system depressurised were not discussed as the aircraft approached the stand and there was no evidence to suggest that this aspect had been considered previously. The first officer brought the aircraft to a halt on Stand 23 using normal braking; the APU had been started. The commander stated that he then waited, with the engines still running, for the ramp engineer to plug in the external power and to chock the wheels. The ramp engineer, standing on steps, moved to plug in the external power connector, however, chocks were not placed under the main wheels because, for safety reasons, it was not the normal procedure to do this until the engines had been shut down. The commander, assumed that the chocks were in place and, after a brief discussion with the flight engineer, he applied the parking brake. Meanwhile, the flight engineer used the push button, on the left side of the overhead panel, to activate the AC electric pump and pressurize the Yellow system accumulators. The action of pulling the parking brake lever de-activated the normal braking system, however, there was insufficient pressure in the Yellow system to activate the brakes and, as the ramp had a down slope and the engines were still running, the aircraft started to roll forward. The ramp engineer dropped the power connector, jumped from the steps and vacated the immediate area; he sustained minor injuries. The commander was immediately aware that the aircraft had started to move and his first action was to recycle the parking brake lever. This had no apparent effect and so he returned it to the off position in order to re-instate the normal braking system; both the commander and the first officer applied pressure to the foot pedals. This was probably coincident with the flight engineer’s decision to select the alternate brake system using the switch to the left of the AC pump push button. As there was still insufficient pressure in the Yellow system, the foot pedals were now ineffective and the aircraft continued to move forward. The commander ordered both engines to be shut down. The flight engineer reselected normal braking and there was some evidence to suggest that, although the crew were not aware of it, a short period of braking did occur. It was, however, too late to prevent the aircraft impacting the terminal building.