Harry Christophers has a refreshing way of exploding expectations. We've scheduled an interview for a break during a rehearsal at St John's, Smith Square, which is the unofficial chaplaincy to Conservative Party HQ, a stone's throw away. Christophers and the Sixteen - the early music choir he founded in 1977 - are approaching the end of their first major English tour, and I go along expecting a live, private sampler of the programme.

But there is no rehearsal in progress, and more disturbingly still, the aisles of St John's are thronging with hundreds of very well-dressed, rich-looking people knocking back large glasses of white wine. Christophers steers his way through the crowd, in full evening dress, wine glass aloft, and suggests we escape to the Box Office located in the far less atmospheric setting of the Crypt. He apologises for the mix-up. "A memorial concert for Sir Robert Sainsbury (the supermarket magnate). Just ended. He didn't want a religious service. A really remarkable man."

So is Christophers: a slim, very thoughtful 40-something with a lined, unmistakably kind face.

I ask about the group's rather grandly-titled tour: "A Choral Pilgrimage: Inspiring Music in England's Finest Cathedrals". Although I am too cowardly to tell him so, I've already got it in for that title (faux, goody-goody, nearly as bad as "These You Have Loved"), and a snippet in the accompanying programme notes has made me want to go for my gun:

The dark descends early in these northern climes. Candles are already lit, for even- song, as we hurry through the south, or the north door (the great west door is rarely opened, saving the visionary length of the whole cathedral for special ceremonials) to escape the cold.

There's no harm in it, I suppose, and it's faintly amusing in a camp sort of way. But it's a bit much to be asked to take it seriously. Christophers's own manner, mercifully, is a world apart from these sentences. "The idea behind the tour," he explains briskly, "is to perform the music of pre-Reformation, 16th-century England in the churches where it would have been first heard. It's a part of English musical heritage that was nearly destroyed by the Reformation."

So the candles had their point after all.

"In some areas, architecture for example, what's ruined can be recovered, though that usually takes a generation at least. But when a work of art is destroyed it can never be replaced. Before the Reformation, every English cathedral would have had its own part-book containing works painstakingly built up for use by its own choir. Only three of those books survive. We'll never know how much great music was destroyed or indeed whether there were great composers that have been forgotten unnecessarily."

He pauses as if to take in afresh the gravity of this fact. But then another thought displaces it and a broad smile appears: "But it's staggering how many gems have been passed down to us from the surviving part-books and from manuscript sources!"

You get the sense that Christophers is much preoccupied by how different the course of musical history might have been. "What would Byrd have done if it [the Reformation] had never happened? He went on, of course, composing devotional works in Latin. But these had to be performed in secret, in the homes of recusant Catholic nobles like the Earl of Northampton and Lord Petre of Writtle. But you sense that frustration there, all the time.

"He might have gone the way of [Tomas Luis de] Victoria," Christophers's favourite continental composer of the period. "Victoria was everything. He was a musician, a scholar, a thinker, the lot. And he anticipates the whole power of opera. We've been going through reams and reams of his work and there isn't a bad bit in it." He pauses anxiously before laughing warmly at himself: "No, wait. If it's bad it'll be our fault."

Christophers and the Sixteen are doing two concerts at the Belfast Festival, one in each cathedral. At Saint Anne's, there will be a chance to hear Benjamin Britten's rarely performed Sacred and Profane: "It was one of the last works Britten finished. We've been doing it for a bit now and recorded it a few months ago. It's fiendishly hard but very rewarding for the listener." The programme will also include works by Byrd, Tallis and John Tavener.

At the performance the following night in St Peter's, the programme will be exclusively 16th-century. On the programme are two settings of Quo Modo Cantaribus by De Monte and Byrd, which came about as a result of a correspondence between the two men. "It very much shows who the master is!" says Christophers. (Byrd, of course.) There will also be works by Guerrero, Victoria, Tallis and Sheppard.

How does he feel about performing sacred music for secular audiences? "Some people," he says in a tone which indicates his distance from them, "mostly scholars, actually believe that this sort of music requires all the trappings of a 16th-century church ceremony. So you should only sing it in places like Eton College Chapel. And you should aim to reproduce not only the sound of the music as exactly as you can but also the ethos underpinning it. We don't. We want to bring this music out to people."

BUT isn't authenticity what groups like the Sixteen are all about? I ask. "No it isn't. Not any more. When we began back in 1977, I suppose it was. We wanted to perform the music authentically," he says, laughing as he realises he's been talking as much about himself as about academic purists. "But as we grew to love it, we saw how important it was to champion it as well. And you can't champion this music authentically.

"You can't do it in the terms in which it was championed in the 16th century. The works of Tallis and Byrd and Victoria were performed before audiences who knew every line of the Latin mass. That's not the case any more. Now you have to convey the spiritual dimension of the music by tone-painting, by singling out musical intricacies that might otherwise elude the listener. That's what we do. Some people might think we go too far." Then he delivers the mischievous coup de grace: "What we do now is wholly inauthentic."

He's obviously proud of that.

The Sixteen performs at St Anne's Cathedral on Thursday, November 2nd, and at St Peter's Cathedral on Friday, November 3rd at 8 p.m.