Please be aware that there is fault-finding process that leads to reasonable confidence of determining which part has failed. Fault-finding is rarely easy for a novice, even with the kind of advice you can find on this forum.
Having said that, if an assistant opens a distant hot tap while you press your ear to the boiler, can you hear a "click"? You should hear one, because this is the noise that the flow switch makes. If you don't hear the noise then it's quite likely that the flow switch is faulty.
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