Worcester bosch 28si ii wont fire up once hot?

Hi peeps not long moved into my first morgaged home to find out that the boiler does not ignite when hot, can have the heating on for 2-3 hours but once it goes off it fails to start had a company out and they think it is the pcb or flame detection sensor but while he was checking he found that the burner doesnot alway light all the way across what could cause this or would getting a new burner fix the problem

Thanks in advance
paulyboi1989, October 2016


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