The left hand fuel filler cap locking tongue had been replaced during the periodic servicing 20 hours prior to the accident. The new tongue did not have a positive feel when pushed to the closed position and the cap did not seal correctly. The pilot was aware that the bladder type fuel cells could trap water which might not be apparent during a normal fuel drain. About one week before the accident and following heavy rain, the pilot rocked the aircraft, during his fuel drain check, to remove all the trapped water. On the day of the accident the pilot refuelled the aircraft, to full tanks, from an un-opened 200 litre drum. A subsequent check of the drum disclosed some water although in insufficient quantities to enter the standpipe of the hand pump during refuelling operations. The pilot carried out a normal fuel drain check which indicated that the system was free of water. He did not rock the aircraft as there had not been any significant rainfall since his previous check. At 150 feet above ground level, shortly after take off and following flap retraction and power reduction, the engine stopped. The pilot carried out a turn through 170 degrees in an attempt to land in the only reasonable area. The nosewheel collapsed on touchdown and the aircraft overturned. An inspection of the aircraft's fuel system disclosed water contamination in the main fuel filter bowl, in the fuel lines to the carburettor and in the carburettor. No other defects found which may have been factors in the accident. It is probable that water entered the fuel tanks, via the defective fuel cap and remained trapped in the tank despite the checks carried out by the pilot. Water subsequently entered the engine fuel system during the manoeuvers following take-off.