Will contact our (Oftec registered, amazingly!) heating engineers and suggest they look at replacing the burner.
NO, NO, NO!!!! DO NOT suggest they do anything particular.
In fact, why not get someone who can sort it? Oftec registration means they are oftec registered, it doesn't make them any good, just as not being registered doesn't make them bad. Though in fairness if the fault is intermittent, they are absolute pigs to sort.