Hi, could be one of many things!! Thermostsat stuck, radiator blocked, water pump failed or head gasket problems, the later being the most costly!! is there any mayonaise like gunge in the oil filler cap and in the rocker cover as this would tend to indicate a gasket problem, any good independant garage should be able to carry out a "sniff test" of the water in the expansion tank if the sniff test fluid changes from blue to yellow then it's bad news
nc
August 2009