Following an extended flight over water, the helicopter suffered a power loss as it was approaching the coast. The pilot conducted an autorotational descent and landing. The landing on coastal mud flats was heavy, resulting in damage to the skid gear and tail boom. Examination by a Licenced Aircraft Maintenance Engineer (LAME) found that there were blockages of both venturis in the fuel transfer system which prevented the flow of fuel from the two forward tanks to the main fuel tank which supplies the engine. The engine had been running following the forced landing. The fuel supply was obviously not blocked completely, but suffiently to prevent operations at normal flight power settings. The material blocking the two venturis appeared to consist of tiny specks of fuel tank liner.