Over the next few days marquees will be popping up in various places to serve as prayer venues for the High Holy Days. But the canvas synagogue at Moishe House London in Willesden will have one unique feature; it must be the only one to contain a "trichitzah", a three-way mechitzah with sections for men, women and mixed seating.
Moishe House is hosting Grassroots Jews, the alternative High Holy Days services launched two years ago by a group of young people looking for something different from conventional synagogue fare. As well as the traditional prayers, there will be a parallel programme of meditation, discussion, improvised drama and other activities.
On Sunday artist Rutie Borthwick and others taking part will put the finishing touches to the trichitzah and design other artwork to adorn their makeshift spiritual home at an "art and learning" day. That preparation is typical of the do-it-yourself philosophy of the independent minyan movement.
For Grassroots is more than once-a-year phenomenon. It is the umbrella term for a loose coalition of groups that have sprung up across London over the past decade, offering Friday night gatherings like Wandering Jews or the Carlebach Minyan or events ranging from an environmentally themed Shabbaton on a farm to a Shavuot retreat in Tuscany.
At the cutting-edge of contemporary Judaism, independent minyans echo the chavurah movement of the 60s, founded by young people impatient with synagogue bureaucracy and denominational divisions and willing to experiment with tradition.
Rutie, who is 28 and was raised in one of London's largest Reform communities, said that "most organised synagogues feel very institutionalised. In Grassroots, you feel you can have a say. It's a very accepting place where you meet people from all different backgrounds and different Jewish connections."
Its diverse constituency spans secular to Orthodox. But one of the challenges that this emerging community grapples with is trying to find a modus vivendi that everyone can live with.
Grassroots co-ordinator Rachel Marcus, an Oxford-educated environmental lawyer aged 30, is herself from a committed Orthodox family who studied at a seminary in Israel in her gap years. When it came to planning the High Holy Day services, she said, "It was very important for us that those who consider themselves within the spectrum of Orthodoxy would also be able to feel comfortable."
She was drawn to the independent scene after arriving in London after university and finding "nothing that really got me excited Jewishly." At mainstream synagogues, she found there was little to contribute "unless you are a member somewhere, you send your children to cheder, and unless you are a man. If I was counted in an Orthodox minyan, I'd be there every morning."
At Grassroots, she and her peers have quietly been extending the boundaries of British Orthodoxy, following the example of such communities as Jerusalem's Shirah Chadashah, which allows women to lead prayers in certain parts of the service. At Rosh Hashanah last year, she said "We had a debate whether a woman can blow the shofar for men who don't think their obligation would be fulfilled by listening to it." (In the end, shofar blower happened to be male).
"Obviously, the way we do things at Grassroots wouldn't be acceptable to everyone within the Orthodox fold," she explained. "We are still feeling our way. Ad hoc decisions are made which I am not necessarily happy with halachically - they are made on the day because something comes up and we'll think about it for the next time. But it's not the end of the world
"If a decision is made on the cuff which someone is not happy with, they are not going to walk away from the group. That goes for someone who feels their Orthodox, or their feminist, or their socialist, sensibilities have been affronted."
While she is a self-confessed "halachah geek", she said, "there are plenty of people who come along who are very spiritual but who don't believe in any way in the halachic framework, as well as people who are not at all spiritual but committed to the cultural and community aspects."
But still the long-term question remains whether Grassroots will turn out to be a passing trend. Rachel - who is marrying Gefiltefest founder Michael Leventhal at the end of the year - is optimistic that the "independent minyan genre is not going to go away. People get so much more out of davening in community, and because they feel they want to rather than any pull of obligation. And because they are able to lift up their voices without restraint - I don't see that anywhere within the establishment communities."
The numbers involved in Grassroots still remains small, points out Rabbi Shoshana Boyd-Gelfand, the director of J-Hub, the innovation centre which incubates new Jewish groups. "It is going to attract a small elite group because it takes quite an investment to create your own minyan. You need a knowledge base how to lead services or give a dvar Torah."
Will they, as others have done, simply be swallowed up by the existing synagogue structures or will they become agents of change that might help transform congregational life more generally. "When they have kids, will they sit back in synagogue and slot into place," Rabbi Boyd-Gelfand asks. Or they will be the kind of people who will say "'I don't just want to send my children to cheder, I want to create family retreats'. The question no one knows the answer to is what will happen to that core group of people in 20 years."