The pilot had not flown for about three weeks and was conducting his first Hang gliding flight from the site. The site was advanced rated and the wind conditions were light and variable with occasional thermal gusts. The pilot was assisted in setting up the glider but declined launch assistance. He appeared anxious to begin the flight and was not seen to perform the mandatory hang check to confirm his attachment to the glider. Following a good launch the glider was observed to sink rapidly and fly in a straight path towards a large tree some 30 metres in front of the launch site. The glider struck the tree trunk and the pilot was thrown clear. Examination of the equipment revealed no damage to the harness support system and no other pre-existing fault or damage. The flight path observed was typical of what would be expected if the pilot had failed to connect his harness to the glider.