From the minute I walked through the door at Waxwell," Mary Grasar explains, "I knew it was where I wanted to be. I can't explain it any more than that. I just knew." She smiles before adding, "That was more than 40 years ago now." As she talks, Grasar is leading the way through Waxwell's eight-and-a-half acres of grounds, tucked away behind housing estates and suburban sprawl in the unlikely setting of Pinner. It may be only a dozen miles north-west of the centre of London but, protectively wrapped in overhanging branches, blackberry bushes and birdsong, we could just as easily be buried deep in the silence of the English countryside.

The long grass, coated in autumn dew, is coming up over Grasar's open-toed sandals. Behind us are the formal gardens that surround the original Elizabethan house and the mellow, mock-rustic Edwardian wing that, together with a more modern accommodation block, a chapel and a round, 1960s meeting hall, make up Waxwell. It is home to the Grail Community of Catholic, celibate women.

It is a description that makes them immediately sound like a convent of nuns, but the Grail's 10 members would disagree. "I've never had any wish to be locked away," protests the community's oldest member, 98-year-old Philippa Craig. "When I joined in 1935, I had an aunt who was a nun. She sat me down and asked why I was making a commitment to something as 'half-baked' as the Grail. Her choice of words only made me more determined."

Ahead of us is a labyrinth, cut into the knee-high meadow for prayerful walking. Beyond it lie beehives and a tree-lined dell where, Grasar explains, there was an outdoor altar until a tree fell on it. Alongside us are two large, pretty wooden cabins, surrounded by flowers and tucked under the trees.

The smaller one is called Walnut and the bigger one is divided into two parts: Apple and Apricot. All are poustinias, or hermit's cells. They contain the bare necessities of life – a bed, a table, a chair, a simple bathroom. There are a number of others of various shapes and sizes dotted around the grounds of Waxwell, their windows peeping out from the bushes like the eye of a bird.

The poustinias – found first in the Russian Orthodox tradition – are an escape, made available by the Grail to anyone who has a use for them, from whatever background, religious or not. The only essential is a wish to get away from the world, be it for a few days or a few months. By letting them to all-comers, the Grail Community hopes poustiniks will not only commune with nature, but with all that is sacred.

"When I first joined, all of us in the community used to have time for ourselves in one or other of them," Grasar recalls, "but not so often now." The cabins, as I can see when I peer into Walnut, are clearly still well used. Institutional religion may be on the wane, but the search for the spiritual is currently very popular in these uncertain times. So why is there a note of regret in Grasar's otherwise warm, upbeat voice?

A bishop's niece in her sixties, direct and unadorned, she worked as a primary-school teacher before joining the ' Grail Community as a young woman in 1966. Her decades of commitment pale next to some of its other residents. As well as Craig, there is Betty McConville, a short, smartly dressed 86 year-old, who was one of the first two Grail members to move into Waxwell when it was given to the community by a benefactor in 1947. "I was dumped here with two camp beds and two chairs," she says. "That is all there was back then." Catherine Widdicombe ("no relation"), also now in her eighties, arrived soon afterwards.

All have grown old, surrounded by this heavenly garden (the origins of the word paradise lay in the pairidaeza or the "beautiful walled garden" of ancient Persian kings). And all – like anyone else who sets foot in Waxwell – have developed a deep bond with the place.

Yet, as a shrinking and ageing community (down from a peak of 25, with the youngest current member in her mid-fifties, and six of the 10 being 80-plus), they are confronted with a reduced capacity to manage the place, the disproportionate demands made on the more able members to the detriment of their other work, and the struggle to pay for its upkeep. "We have decided," says Grasar, "that, if the Grail Community is going to have a future, we must move to somewhere smaller." An anonymous buyer has been found – someone sympathetic to what has gone on here for so long and willing, therefore, to allow a period of grace before taking possession. The clock is ticking. "I find the thought horrendous," reflects Grasar, "but at the same time I feel it is the right thing to do and a challenge to us all."

We tend to associate radicalism with youth, and when the Grail started out in the Netherlands in the 1920s, it combined both. Inspired by a Jesuit priest, James van Ginneken, it was a movement for young Catholic women with no interest in being nuns and who therefore faced being shut out of making any active contribution, much less taking a leadership role in their church. The Grail uniquely offered the chance to join a community where they would support each other as they dedicated their lives to educating the laity, especially young women, within the church, and working more broadly for social justice. It was an immediate success and by the 1930s had come to Britain.

"I had just left art college," recalls Craig, "when the girlfriend of one of my brothers told me about the Grail. I went along to a meeting and got to know the leaders and found it inspiring. So I thought I'd help out for a bit. They had a Grail 'club house' in Islington, which was back then a very poor area indeed, and I ran some sketching classes for local people." Once there, she found herself increasingly drawn to the Grail's combination of social work, faith in action, and community living, and all of it possible while still wearing everyday clothes rather than a religious habit.

The present-day community is gathering in the panelled sitting-room at Waxwell. Sitting around me is a group of accomplished, highly articulate women. Most have achieved distinction in their professional lives. Craig, for example, was one of a celebrated team who translated the Psalms into modern-day English. Widdicombe co-founded Avec, an agency to promote collaborative ministry in churches when such inter-faith initiatives were rare. McConville is a counsellor and used to stage plays for the Grail (adaptations of such spiritual classics as Francis Thompson's The Hound of Heaven) at the Royal Albert Hall.

Though no longer young, they quickly demonstrate as we talk that they have lost none of their radical edge. The Grail may be formally linked to the Church (as a "secular institute"), but they are independent souls, painfully aware of the shortcomings of the male, clerical culture that continues to dominate Catholicism, and the secondary role it still allots to women. Most are in favour, for example, of female priests – although none appears to feel that particular vocation themselves. "I think we'd have to change our ideas of what a priest is before I'd want to become one," McConville remarks politely but pointedly.

When, all those years ago, Craig's nun aunt labelled the Grail "half-baked", she meant it as an insult, but, to those who like precision and prescription, it does lack definition. On one of its leaflets, for example, it says: "Grail people do not stick out in a crowd. They look no different from anyone else but if you want them, they'll be there for you."

This open-ended offer of service remains at the heart of the community, and the flexibility it affords, they believe, has helped the Grail to keep pace with a changing world and changing needs. "In the traditional religious orders, they are either all about teaching or nursing or contemplation," says McConville who started out working for Boots in Nottingham before joining the community. "But we have an adaptability that made me feel from the start that I could contribute something, that I could have a value. And it wasn't pious. My older sister was a nun and religious life did not attract me. I suppose what the Grail said to me above all was that ordinary life was important and valuable."

That rootedness has drawn others. Moira Leigh, a youth worker now also involved in the maintenance of the property, was also a primary-school teacher before she joined in 1972. "When I first visited Waxwell, I found a God-filled place, but what I also remember very clearly was seeing two members of the community arguing over a hosepipe. I realised that these were real people. Maybe I could be one of them."

And so the Grail has continually adapted – from its own strict rules of conduct in the 1940s, for example, to the more democratic approach to community living of today. Some principles, though, remain unchanged. Funds are held in common, with each member receiving an allowance. All, though, deal with family inheritances as they see fit; it doesn't belong to the community.

But what about the stipulation that they must be celibate? Isn't that just the sort of outdated idea that the Grail should ditch? "Our dedication calls us to be celibate," replies Grasar, "so allowing us the freedom to be at the service of those whom we minister and to the community. If married, our primary commitment would be towards family."

What has certainly changed radically is how the Grail Community lives out its mission in practical terms. Once they were turning out popular versions of papal teaching documents, translated into everyday language, from a property they leased in Sloane Street in Knightsbridge. Today, the room upstairs at Waxwell that is known as the "Publications Office" is largely silent. Again, there are still retreats and silent prayer meetings held at Waxwell, but for small groups, whereas once its "Family Weeks" and lecture series would have seen several hundred people milling about the grounds.

Has something vital been lost amid this willingness to adapt? "There is still our underlying discipline," insists McConville. "And it can still make you fly. The problem is that when you have been in a vanguard of change, then that change is achieved, to some degree, it leaves you feeling ' a little at sea." It needs a new generation to take up the challenge – to fill Waxwell afresh – but they just haven't come.

In their absence, the community today directs its remaining energy towards simply being there, and being available. So they operate an open-door policy. At any one time, there will be individuals staying with the community – some more formally as short-term members – who are going through crises, traumas and life-changing experiences. "We've had a very strange collection of people here over the years," says Craig. "Carmelite nuns, an Irish priest trying to end a relationship, and the woman with whom he was having the relationship. She followed him over."

And then there are the poustiniks. There is a hot meal available for them each lunchtime in Waxwell's dining-room – with a small bar tucked neatly into the fireplace – but no obligation to attend. Likewise, the community's morning prayer session in the chapel – a bright, open, modern space – is there if the temporary hermits want to join in. And if they want to talk about whatever is troubling them, there are plenty of well-trained ears at their disposal, but not the least element of compunction.

"It comes back to the motive behind everything we do," explains Valerie Wright, a South African in her sixties, and the most recent arrival, in 2001. "We are not trying to set an example to others, and definitely don't see ourselves as examples. We do what we do because we are called to be like Christ. If it has a beneficial effect on anyone else, that is nice, but it is not the point. We would do it regardless."

Wright is a practical example of quite how flexible the Grail ideal has proved. She is both a divorcee and a Methodist lay preacher. Because she has no wish to convert to Catholicism, she cannot become a full member, in the strictest sense, but she is in everything but name. "It is the shared values we have that make this a community," she suggests.

Her grown-up daughter lives in Britain and was initially puzzled by her mother's choice of lifestyle. "I'd probably been here a year when she accused me of giving up on life, turning my back on the chance of a new relationship with a man, and hiding away in a convent. I don't think that was true. I hadn't turned my back, but I just wasn't looking. Some time later, though, when my daughter had started coming here more often, and even staying with us, we were all sitting at supper. Everyone was talking and laughing, and my daughter turned to me and said that she had now realised why I was doing what I was doing, that this wasn't a second-class choice."

So why, if the attraction of the Grail Community became apparent to that young woman, have others not come along to join? It is, of course, a tough climate for any religious organisation, however unusual, trying to attract vocations. The group has clearly discussed this one before, and they offer several explanations. One is that the Grail has chosen over the years to maintain a very low profile, so people just don't know what it is, and how different it is. "We've never made a fuss, partly so the bishops have always trusted us and let us get on with it," reports Widdicombe. "And we've never been strident, or feminist. That's not our line."

"But it is very frustrating," McConville interjects. "Sometimes, when I am at a low ebb, I do wonder what it's all about. Is it worth it?"

"I always think that essentially all of us here value our space," offers Grasar candidly, "and need just to be. So living in community, independent but interdependent, has, I believe, enabled us to do what we might not have done on our own. But nowadays, I think that is changing. It is easier for women to do all the things that we have done, but in their own place. There are women lecturing in theological colleges, for instance, women working as pastoral and outreach workers. When I came here, you just couldn't do that as a woman – or the only way you could, was by being a member of a supportive group within the church like the Grail."

That sense of time leaving the Grail Community behind remains when we return to the garden later. I spot Widdicombe, elegant in a purple blouse and skirt, picking flowers and berries to decorate the communal rooms. It reminds me of something McConville had said earlier: "We have never been austere. There has always been that element of beauty so that Waxwell is a place people can come and relax and not feel as if they are in a religious setting."

I follow Grasar down another path, past another row of cabins. "This one," she explains,"used to be the weaving hut. We had a Swedish women called Ingrid living here who taught weaving. That drew a lot of people in, but she's been dead a long time now." Further on is a row of tiny poustinias, hardly bigger than beach huts. Their roofs are sagging and taped to the window is a notice: "Hard Hats Must Be Worn At All Times". "These were here long before I arrived," says Grasar, "and now they've reached the end of their life. They are no longer safe." The end-of-an-era feeling is welling up.

Will they have poustinias wherever they move to? "I hope so," Grasar replies. "It depends on what we can find. We've been looking at old B&Bs, but they tend to have very little communal space. Or some old religious buildings, but those long, cold corridors..." Not very Waxwell.

If the Grail Community itself is shrinking, there is, it should be pointed out, plenty of life in its two sister organisations – the Companions of the Grail, made up of celibate Catholic women who have their own homes but try to live out the service-to-the-community ideal as individuals; and even more markedly in the growing number of Grail Partners – married couples, again regular visitors to Waxwell, who draw inspiration from the core community and its work, but apply it more widely in the midst of everyday lives and families.

Talk of renewal, then, as opposed to managed decline, is not so far-fetched. And there remains an energy and a contemporary appeal in the Grail approach. It combines two very of-the-minute elements: the search for what is loosely called spirituality, as exemplified by the poustinias, and the urge to give practical expression to loving your neighbour. The Grail has been trying to do this long before David Cameron began talking up the "Big Society".

As the custodians of this history, the ageing but inspiring women of Waxwell are determined therefore that their move will not be the end, but a new beginning. They talk with conviction about their excitement at what lies ahead and the challenge of embracing another change, another adaptation. It will, though, mean saying goodbye to this extraordinary place. "Part of what we are, as individuals and as a community," says Grasar as she stands dry-eyed on the lawn at Waxwell, looking back at the buildings, "is in these stones."