I recommend you to read this article written by an English Roman Catholic priest who celebrated the Sarum Use in Merton College Chapel in Oxford in the 1990's. It would seem, in the light of the evidence presented and the existence of the motu proprio Summorum Pontificum of Benedict XVI, that the Sarum Use is "legal tender" in the Church, just as legal as the Ambrosian Rite, the Roman Rite of 1962 or the Dominican Rite. Is continuous use required? I would think not, since the Ambrosian and Dominican rites ceased to be used for a time, and Paul VI had the intention of replacing the traditional Roman rite with the Novus Ordo without canonically abrogating the old rite.
I am not a canonist, but having read Count Neri Capponi's analysis of the status of the Tridentine rite in the 1970's, I am not difficult to convince about the legitimacy of Sarum as a "current" Catholic rite. Indeed, I think it should gain wider recognition as being a part of Anglican patrimony.
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