During a short ferry flight to a property for the purpose of conducting shooting operations, the pilot noticed a burning smell in the cockpit and a glow beneath the rear seat. Following an emergency landing the pilot evacuated the aircraft before it was consumed by fire. The pilot reports that he had been carrying signal flares, fuel, oil, grease and dirty rags on and beneath the rear seat. He further reports he had been smoking prior to becoming aware of the burning smell. The Fire Officers Investigation report confirms that the fire had originated in the cockpit and that the source of ignition had probably been a lighted cigarette butt which had come to rest on the dirty rags.