Well everything else is going up in price, so Zen's costs must be increasing too. Also, from one of the links you kindly provided, "the provider does still offer a Zen Contract Price Promise that says the firm will not change the monthly costs of a customer's broadband plan during their minimum term contract (unless required to do so by a regulatory body).
What this means is that Zen broadband customers will sign up to a monthly price that will remain the same during their contract, although it will likely rise after the minimum term.
Only a few providers fix their broadband prices for the entirety of a fixed contract, so Zen is still a good option in that respect."
