I have another recording of Britten's Festival Te Deum, by the choir of St. John's College Cambridge in their chapel, it's a Naxos recording. Now the two recordings couldn't be more different. In the St John's one you can definitely hear many things more clearly, you can't hear the hiss of the organ pump, there's virtually no reverb from the chapel and everything is crystal clear. It was probably recorded with dozens of mics. However, it might as well have been recorded in a studio for all the impression it gives you of a live event.