During a pause, one of the singers asks which paper I write for, and grimaces when I reply; his allegiance to The Independent recently received a near-fatal blow because our reports on the Westminster Abbey rumpus favoured the ousted Martin Neary. "You were wrong," insists the singer. "Neary had to go." It turns out that several more of Christophers' choristers sing in the Abbey, while others sing at St Paul's. In the bosom of the Sixteen, you're at the heart of the choral establishment.
The Sixteen's name was once perfectly apposite: 16 singers raked together at Oxford for one concert of 16th-century music. When I drop in there are 18; they go up to 36 when the occasion demands. All are freelances hired by the gig, but six of the founders are still in place after 22 years, and those who have gone on to glittering solo careers still regularly come back to sing.
And though their repertoire has roamed far beyond its original Tudor confines - their Prom this week consists of Poulenc, Durufle and Faure - that repertoire remains exclusively sacred. How important is it to be a believer, I ask Christophers, when you sing to the glory of God? Belief can definitely help, he replies; he himself is a Christian. But the crucial thing is to love the music, and to want to convey its spirit.
Ironically, he says, Catholic choirs get closer to that spirit than Anglicans do; their singing may be rough at the edges, but it's imbued with raw emotion, whereas Anglicans get hung up on precision. Quite so; I once heard the choir of King's College, Cambridge intone "When Israel came out of Egypt" in such a preposterous way that the final word had four syllables, with the "p" and "t" given a syllable each.
King's, Cambridge! In Christophers' book this celebrated choir, led in its heyday by the celebrated Sir David Willcocks, turns out to be public enemy number one. "They've done awful damage, making the music clinical to the point of castration. Our job is to put the heart and soul back into it, to find the rough edges again. In one sense we're all to blame - we and the Tallis Scholars and the Taverner Consort - through our too- perfect CDs. That's why we want to be heard in concert. Live we offer more than our discs can ever do."
He sees his choir as custodians of a grievously eroded musical heritage. "Huge amounts of music were destroyed in the middle of the 16th century," he says. "And of what survives, only a handful of pieces are in the cathedral repertory." This is partly because of clerical reluctance to countenance any work lasting more than five minutes, and partly thanks to the drive to modernise the liturgy. Hence the "Choral Pilgrimage" that will be the Sixteen's contribution to the millennium celebrations.
Is it a response to the music world's equivalent of the Greenwich Dome - the "Bach Cantata Pilgrimage" on which Sir John Eliot Gardiner is taking his Monteverdi Choir next year, singing every one of Bach's 200 extant cantatas all over Europe? "Absolutely not," says Christophers firmly. "I'm not in that league of megalomania." The Sixteen's more realistic pilgrimage - York to Canterbury - will take them through 12 English cathedrals.
In 22 years Christophers has had to sack only two singers, but he's hot on the "right type of voice". "They mustn't get too big in their projection; they have to be able to deliver a particular `straightness' of sound. And above all they must blend - if a voice doesn't do that, it's useless to me. Four singers who blend are more powerful than eight who don't." The sound produced by the BBC Singers, good though they are, is the antithesis of what he is after, verging as it does on the operatic. His worst-ever conducting experience was trying to get an Italian opera company to sing Bach's Mass in B Minor. "Horrendous."
Turnover in the Sixteen is slow, and when Christophers does need new blood he finds it through recommendation rather than audition. He looks above all for what he calls "quick musicians" - sight-reading being the crucial skill - and is prepared to let voices take their time to develop. Female altos, plentiful in Germany and Finland, are in Britain very hard to find. If he had his way he'd make the boys-only Oxbridge choirs admit girls; that, he says, is how the best choral altos are created. He admits that the sound of a boys' choir has a unique "bloom", but sees this as a diminishing asset.
He loves the echoing acoustic of a large church, particularly for Renaissance music. "Composers such as Victoria and Tallis knew exactly how to exploit the sound-boxes they were writing for." So what is his view of the acoustic at the Albert Hall? "Appalling, particularly for Baroque music." How does he cope with it? "You have to forget it. The mistake people often make is trying to compensate by over-singing. You just have to do your normal thing, and make the listener adjust." Or find the right spot? If he were listening to his own concert on Thursday, where would he sit? "I'd be down with the promenaders." So there you have it.
The Sixteen at the Proms tomorrow night will be broadcast live at 10pm on Radio 3