The aircraft was being readied for a flight from Coventry , U.K. to Dublin, Ireland. On board were the flight crew of two and approximately 4,600 kg of freight. The flight crew carried out the pre-start checklist and then started the right engine, after which the ground crew were given the signal to disconnect the ground power unit (GPU), positioned directly in front of the aircraft. The commander expected to see the GPU being moved clear of the aircraft, but when this did not happen he signaled the left engine start to the ground crew, which was approved. With the left engine started, the commander signaled for the nosewheel chocks to be removed, which was actioned by the ground crew. Immediately the chocks were removed, the aircraft began to move forward and the first officer saw the ground crew member run clear of the aircraft to the right. Both flight crew members immediately applied wheel brakes, but as they did so, the aircraft collided with the GPU. The flight crew shut down both engines and informed ATC. There were no injuries and damage was confined to forward parts of the aircraft and the top cover of the GPU. After the accident the flight crew realised that the parking brake was not set, resulting in the aircraft’s movement when the chocks were removed. The commander noted that there had been an expectation that the parking brake would have been set to on when the crew first arrived at the aircraft. The parking brake was not an item on the turnaround checklist which the crew had carried out earlier, although it was an item on the pre-start checklist which had therefore not been carried out correctly. The commander also observed that the signal to remove the chocks was given before the GPU had been moved clear of the aircraft. Although the flight crew had not felt rushed, the commander believed that they had been under a time pressure due to the imminent airport closure, and that this had been a contributory factor.