The pilot reported that the power checks were normal; however the takeoff roll was a little longer than expected and the rate of climb was less than 400 ft/min, so the decision was made to return to the airfield. A smell of burning was then noticed in the cockpit, with white smoke emanating from the cowling. At around 500 ft agl, the engine started to lose power and the smoke increased in intensity. The pilot made a MAYDAY call, switched the fuel off and completed a successful forced landing in a field. The occupants exited through the two doors, and the pilot returned to the aircraft and used a fire extinguisher to fight the flames in the engine bay.
The pilot believes that the exhaust had become detached and the hot gases had started a fire in the engine bay, which also caused damage to the lower fuselage.
The pilot had an instructor rating and hence had good recency in practice forced landings. This, and the fairly low height when the fire started, were probably factors that resulted in a successful forced landing.