At Kol Nidre this year, a visitor would have been able to walk into Hendon United Synagogue in north-west London and comfortably find a seat. Twenty years ago, for one of the 25 biggest congregations in the country not to have been full would have been inconceivable.

While the main service could still boast the pomp of chazan and choir, elsewhere in the building two alternative minyanim catered for a younger age group, where Chasidic-style niggunim were more the more likely musical order of the day. But this is not something you will see just on High Holy Days. Go to Hendon, or many other synagogues on a Shabbat morning, and there will be a variety of services.

The alternative minyan has become a standard feature of 21st century worship. Big "multiplex" synagogues can offer three, even four different minyanim on a Saturday, not to mention youth and children's services. The biggest Masorti synagogue, New North London, hosts one traditional (separate seating) and two egalitarian minyanim.

When alternative minyanim first emerged in mainstream synagogues 20 odd years ago, they were sometimes seen as divisive and accused of factionalism. Finchley United Synagogue once almost split over its alternative minyan, but today, under its rabbi, Ephraim Mirvis, it has become a model congregation whose repertoire even includes a Sephardi service. The threatening trend is now hailed as a sign of progress.

"Many years ago the view was that everyone had to be in one service," said US president Stephen Pack. "Now we are much more concerned with how many people come through the door, and davening where they feel comfortable. When rabbis realised that's what mattered, they began to encourage it."

The alternative minyan is typically more informal than the "high synagogue" style of old Anglo-Jewry and more intimate - where "everyone knows your name", to quote one regular. New tunes from Israel or the USA prevail over the more sonorous 19th century melodies of Minhag Anglia. And, significantly, they are more participatory, led by members rather than by chazan or rabbi. "People want to get more involved and be more hands-on," said Mr Pack. "I think that's a great trend."

People may often identify more with the minyan they attend rather than the larger synagogue, but Mr Pack is relaxed, observing: "Everyone can come together at kiddush".

These multiplying minyanim, in a sense, represent the stiebl-isation of British Jewry, the preference for smaller prayer venues that is a hallmark of the burgeoning strictly Orthodox community. New boutique synagogues have sprung up to meet the demand for niche worship. The Village Shul, an independent Orthodox congregation of 50 families, founded four years ago, has recently moved into its new home in the New End Theatre in Hampstead.

But some of the independents have opted to come under the umbrella of established synagogues. The Saatchi Synagogue, founded to attract young singles, now meets on the premises of St John's Wood Synagogue. The egalitarian Assif, a nursery for young Limmud leaders, is based at New North London. Alei Tzion, established in Hendon by committed young Orthodox , recently decided to join the United Synagogue.

For all the challenges of secular life, the synagogue has remained a central institution in British Jewry, with almost three-quarters of Jewish households still affiliated to one, according to the last survey in 2010 (though it depends how you calculate the figures). As the Jewish population has fallen, inevitably so has the number of shul members, by around 17 per cent over two decades.

But overall membership fell by only one per cent from five years before - decline among mainstream communities being offset by the continuing growth of the strictly Orthodox.

In 20 years, the Charedim have risen from 4.5 per cent of synagogue members to nearly 11 per cent. Masorti has almost doubled from just over one per cent to 2.5 per cent since 1990. Sephardim, boosted by a number of small independent communities, have increased their share from just over three per cent to 3.5 per cent. The Reform and Liberals together have gone up from 25 per cent to 27 per cent in two decades - although both dropped slightly in recent years.

The biggest losers have been the central Orthodox communities comprising the US, Federation and their regional counterparts - down from two-thirds of UK synagogue members in 1990, to just under 55 per cent last year. "There has been a drop in 20 years," said Mr Pack, "but our perception is that it has bottomed out. Some of our big shuls are doing well and expanding rapidly."

All synagogue leaders want to engage elusive young adults after graduation. Orthodox outreach groups such as Lubavitch, Aish or the Jewish Learning Exchange, have virtually cornered the market in activities for those in their 20s and 30s, leaving the mainstream synagogue movements in the slipstream. Only in the past few years has the United Synagogue attempted to make up for lost ground with the launch of Tribe, and the Reform with Jeneration.

Meanwhile, a new independent spirit has become apparent in groups like Wandering Jews or the Carlebach Minyan, which arrange Friday night gatherings in people's homes. A new young Progressive group called Leapp – Learning, Eating and Progressive Prayer – recently held a vegetarian Friday night dinner with a service and study of mystical texts. Grassroots Jews attracted 200 participants to its third High Holy Day services this year.

Joel Stanley, 32, a former New North London youth director, said: "There is a myth that Grassroots Jews-style communities are a threat to establishment life. In fact, enabling people to do things for themselves makes it more likely they will get involved in Jewish life, both now and in the future."

Four years ago Mr Stanley was a co-founder of London's Moishe House, a community house of young people who run Jewish programmes for their peers. Grassroots Jews held services in a marquee in the Moishe House garden.

Judith Williams, who heads the Reform Jeneration department, believes groups like Leapp should not be seen as competition for synagogues but as an investment for the future.

"Synagogue affiliation may be down," she said. "But the need to belong has not changed. Is a community always going to look like four walls with a bimah and a rabbi? That is a question we need to ask ourselves. But the need to come together in a way that allows us to connect with other people and to find meaning to life, that is core."