Not all sweetness and light: For 25 years the King's Singers have been our top a cappella group. And for 25 years we haven't had a clue what they're about. By Mark Pappenheim

The launch of the King's Singers must have seemed one of the more mistimed evenements of May 1968. With the world apparently poised on the edge of revolution, and students and workers uniting on every street corner, along came this clean-cut collection of ex-King's College choristers, offering a style of close harmony singing so cloying it could leave the claustrophobic gasping for breath and a choice of repertoire so safe there was hardly a mum in the country who didn't want an album for Christmas and a King's Singer for her son-in-law.

Twenty-five years on, the look and the line-up may have changed - the last two original members are having their farewell concert at the Barbican this weekend - but the sound, and many of the songs, remain the same. Not that all was ever just sweetness and light music for the all-male a cappella sextet. The 'Six Healthy Englishmen' (as an early BBCtv documentary dubiously dubbed them) may well have toyed once with calling themselves Swingers not Singers, and have certainly always been more likely to pop up on Radio 2 than Radio 3. Yet, besides singing almost everything from Machaut and madrigals to Lennon and McCartney and Flanders and Swann, they have also commissioned more than 200 new works from a host of living composers ranging from Luciano Berio and Peter Maxwell Davies to John Tavener (in his Apple days) and Gyorgy Ligeti (whose latest Nonsense Madrigal they premiered at the Huddersfield Festival last month).

That's accepted abroad, in Germany especially, where the Singers are seen more as serious classical recitalists than as a novelty variety act. Here, however, they have never quite managed to live down the MOR image (that's 'middle of the road', for all you under-25s) they earned in the Seventies with all those TV guest slots alongside Val Doonican, Twiggy, Lulu and Nana Mouskouri. Their recordings can't have helped: their most recent releases, under a new contract with RCA, range from Good Vibrations, a compilation of classic pop songs, and Chanson d'Amour, a collection of love lyrics, to Here's a Howdy Do], a Gilbert & Sullivan selection.

But then, as David Hurley (alto 1) says, variety is the group's main attraction. 'That's what makes it interesting to audiences and, more importantly, to us.' It's a sound they specialise in, not an area of repertoire. That said, their latest CD, Renaissance, a collection of the complete six-voice song and motets of Josquin Desprez, is clearly meant as something of a stylistic marker.

As Simon Carrington (baritone 2, and one of the departing founder members) admits, when it came to period style, the group had long been left behind by the onward rush of historical awareness. 'In its day our 1973 album of Italian and English madrigals was something of a trend-setter. But we're not by any means as special in Renaissance music as we were then. On the other hand, we still think the way we communicate it to people who are not specialists is a big advantage.'

Under the influence of new arrivals like Hurley - who joined in 1990 after a freelance career with such specialist groups as the Tallis Scholars and Gabrieli Consort - Carrington believes the Singers have brushed up their technique. 'We've gone for cleaner, firmer lines, though whether anybody will notice, I don't know. But we've tried to learn from the specialist groups and yet produce a sound which is still our own.' As Hurley admits, given the essence of the King's Singers sound, some may still find it a case of the blend leading to the bland.

After 25 years, however, Carrington and his fellow founder member, Alastair Hume (alto 2), knew it was time to go - 'and the silver jubilee seemed as good a date as any. We calculated that we could just about survive vocally until then'. As he says, with around 110 concerts a year, and rarely the same programme two nights running, a King's Singer puts his voice under considerable strain. Not only does close harmony singing demand even more rigorous self-discipline than solo singing, but the group's tailor-made repertoire means no one can take a night off except in the most dire circumstances - 'unlike an opera singer, we can't just get in an understudy'. (He himself didn't miss a concert in 22 years.)

Unlike an opera singer, a King's Singer is also stuck with a particular range and can't just swap roles - from romantic lead to comic cameo, say - to suit his declining vocal resources. 'Al and I are both 51 now and that's getting on for an ensemble singer. And we're still expected to do the same things and produce the same voice we did when we were 30.' Not that the old- timers have been allowed to go gently: with Ligeti and Stefan Niculescu premieres at Huddersfield last month, they have been learning new scores right to the end.

The departure of the last two members of the original Sixties sextet might have seemed like a good opportunity to give the Singers a completely fresh profile. The result is going to be more like a face-lift. As Simon Carrington points out, he is currently 24 years older than the group's youngest member, Stephen Connolly (bass, joined 1987) - 'and I know for a fact that I'm five years older than his mum'. The age span after the two new recruits begin in January will match much more closely the originally youthful make-up of the group.

Each new voice has brought a certain change in direction: Bruce Russell (baritone 1, joined 1987) was behind the Josquin project; Bob Chilcott (tenor, joined 1985) has steered the group towards Minimalism and managed to persuade Steve Reich to write a piece for 1995. But basically, says Hurley, it's 'business as usual'.

And for Nigel Short, who caught the 'close-harmony bug' when he first heard the Singers as a 15-year- old choirboy, the wait is almost over. It's more than six months since he learnt he would be taking over Alastair Hume's role as Alto 2 in January - 'and now I'm just desperate to get on with it'. But with 25 years' repertoire to learn, he's got a lot of catching up to do. 'I'm getting sent new music all the time,' he complains. 'But I've got no time to learn it - I'm too busy putting up shelves to carry it all.'

Lunchtime concert, Fri 1pm St Giles, Cripplegate; 25th Anniversary Concert, Sat 7.30pm Barbican, Silk St EC2 (returns only: 071-638 8891)

(Photographs omitted)