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Hi Petal54
Many thanks for your response, I had an engineer service them last year but all he did was change the nozzle! Since then I have found the list for my boilers of what should be done on a service, so now I am wary of getting anyone out!
I have managed to sort out most all the problems I have had with them apart from this one. This one is giving the same symptons as the other one did earlier this year and I changed the jet and cleaned away the soot and it worked fine, I was well chuffed with my self (as I'm a female!!)