its a job for an engineer, he will test the thermocouple, its most likely that which has burnt out, maybe get the right replacement part ready for his visit to save time, get the gas councill number off the data plate on your boiler to help a boiler spares merchant identify what you need or take as much boiler detail as you can to them, they often want to know the make of gas valve too. a plumbers merchant will give you a universal thermocouple and they dont fit all the ideal boilers
john, January 2010