Ben is also correct in possible causes as he was present when we sorted out my issue. Just an additional word of warning, if the red light blinks in flight but stops after landing and you find the header tank full on landing it might still not just be a faulty sensor but also a restricted breather line. Why do I say so? Well if you pull power on descent for a precautionary, your gravity feed might now exceed the fuel burn which, given enough time, will allow the header tank to fill up again. This will mask a possible kinked feeder tube or restricted breather tube and cause you to erroneously suspect your sensor!
