I am installing a new WC in a flat and the cistern will be fed from the side directly from the adjacent high pressure rising main.  Some web advisers seem to say that connecting the cistern direct to the rising main with a flexible hose is a definite no-no, whilst others say it's jut fine, and that the only problem occurs when the feed pressure is too low.  I'm confused.  Which is it?

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How high? Cistern might be noisy without a pressure reducer.