The aircraft was parked on the aircraft loading area. There were flood lights on the southern side, but the light from these only dimly illuminated the northern side. After engine start the pilot turned on the taxi and landing lights and started to taxi towards the northern side. There was a taxi line along the northern side, leading out of the area in a westerly direction. This line was not seen by the pilot, possibly because he was changing the radio frequency selection from Clearance Delivery to Surface Movement Control. The aircraft left the sealed area, tracking about north-west and ran onto the grass. This was outside the limit of the marked movement area. Realising he was now on the grass the pilot turned the aircraft left, but the right main wheel struck an exposed concrete slab. The right main gear leg collapsed.