Agile - well, what can I say?
You're right, in that that I know more about computers than plumbing, but that's because I know an awful lot about computers. You, however, appear to be struggling with (a) diagnosing the problem on this topic, and (b) understanding who first suggested that the F&E cistern was empty.
Oh, and you have the right, of course, to call a cistern a tank, but it doesn't stop it being a cistern, and it doesn't mean that it is a tank.