You refer to the fan, so I assume you have the aspirated boiler burner with a glow plug and transformer. Are you saying that when the boiler comes on that the cooker burner goes off? If so, that would point to a fuel supply problem or a blocked flue, but that would then cause problems with fan proving and the pressure switch.
When was this last serviced ? I think you need to find someone who knows how these things work.
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