Capel-y-ffin - the Chapel at the End - is a bit like those green valleys in the Himalayas known as sanctuaries, which are entirely cut off by the surrounding mountains. For at least the last 1,500 years, this beautiful, lonely, timeless place has been a spiritual retreat, where the outside world ceases to matter. If it was in India, someone would have built a five-star ayurvedic massage and wellness centre; in the Pyrenees, it would be full of gift shops selling glow-in-the-dark crucifixes. But despite being only three hours' drive from London, Capel-y-ffin is entirely unspoilt.
The narrow road from Hay-on-Wye winds steeply down from Gospel Pass through moorland of windswept bracken, heather, minute wildflowers hidden in tough grass and a few stunted trees of the kind favoured by set designers for stage productions of Beckett or Macbeth. It is populated only by sheep, hang gliders and shaggy but surprisingly elegant upland ponies. Down the hill the trees become taller and denser, overlapping above the road so that it seems as though you are going into a tunnel - the road would quickly become impassable if they were not regularly trimmed back.
Despite the absence of traffic, it is a busy road. The angry couple you soak as you drive through a puddle are William and Dorothy Wordsworth; this was one of their favourite walks. The large, bearded man pointedly complaining about "loathsome British tourists" is the 19th-century curate Francis Kilvert, but you can't get angry with him - you know where he lives (there's now a plaque on the house) and you know all his secrets having read his diary. The boy you accidentally knock off his bicycle is the 15-year-old Bruce Chatwin on his first visit to what he later called one of the emotional centres of his life.
The 11th century Norman warlord William de Lacy was so taken with the place that he abandoned his violent ways and became a hermit here, later founding Llanthony Priory, four miles down the valley. The 12th-century cleric Giraldus Cambrensis commented on the area's "hushed atmosphere", still very noticeable - the acoustics are certainly strange due to the closeness of the hills on all sides. The perspective is slightly odd too, like in an early Braque landscape.
The village at the end has a church, two chapels (one a ruin), a monastery (disused), a Youth Hostel (sadly soon to be closed) but no shop or pub and very few houses.
The monastery and ruined chapel were built in 1870 by Joseph Leycester Lyne, known as Father Ignatius, who tried unsuccessfully to reintroduce the monastic tradition into the Anglican church. He was a mystic, constantly aware of ghosts crowding round him but never answering his questions. Unfortunately, he failed to ask the right questions of his builders who took advantage of his unsuspecting nature and skimped on the foundations, damp proofing, mortar and insulation. The monastic idyll disintegrated along with the buildings, and the chapel is now fenced off with a "Dangerous structure" sign on the door, in far worse condition than Llanthony Priory, which had 700 years' start and the unwanted attentions of Henry VIII to contend with.
Father Ignatius is mostly remembered now because of the events one August morning, shortly after the monks moved in, when three boys connected to the monastery saw a bright light in the form of a woman in a flowing dress. They followed her across the field until she suddenly disappeared. The Virgin Mary, as she was assumed to be, reappeared twice more, although, unlike her later incarnation at the village of Fatima in Portugal, had nothing to say. You can follow her footsteps - probably more of a glide - and every August there is a pilgrimage along the route, the only Marian pilgrimage in the Anglican calendar. A statue of her stands in the place where she disappeared making a gesture that seems to be saying: "Don't ask me, mate. I'm a stranger here myself."
On his death, Lyne left what remained of the monastery to the Benedictine monks of Caldey island, who used it occasionally for holidays (you don't think of monks taking holidays) and it fell further into ruin. In 1924, it was sold to another religious community led by the artist, stonemason and writer Eric Gill.
Gill was a Catholic of independent views. He once said he thought it indecent that men and women should kneel side by side in church. But, according to the 1989 biography by Fiona MacCarthy, he also suffered from - or probably enjoyed - a kind of sexual kleptomania, feeling obliged to have sex with every female he came in contact with, including his sisters, his daughters and, on one occasion at least, the family dog.
The community was looking for a rural idyll and privacy for their somewhat unconventional domestic arrangements, but it was not an easy life, with hard winters and not much better summers, made all the worse by the inadequacies of the house. And it was hopelessly unsuitable for a sculptor as even in the milder months the roads made it almost impossible to move large pieces of stone in and out.
The artist and critic John Rothenstein was an early visitor: "It rained continuously," he wrote in his autobiography, "the house was damp - the paper in my bedroom leaned crazily away from all four walls - there was no hot water, no newspapers, spartan food - and I enjoyed every instant of my visit: Gill's sharp-edged genial talk warmed the bleak house."
While Gill talked, the women of the community had to do most of the work: "the baking and brewing and milking and buttermaking, as well as all the housework, cooking and cleaning, and I," Gill wrote, "no help to anyone, unless you call keeping a general eye on the whole show helpful." They probably didn't, and after only four years the community decamped to the more comfortable surroundings of Pigotts Farm near High Wycombe.
One member of the group was the artist and poet David Jones, who was briefly engaged to Gill's teenage daughter, Petra, until she tired of his diffidence. Capel-y-ffin was possibly the only real home he ever had as an adult; mostly he lived in bedsits, stumbling cheerfully from one nervous breakdown to the next. He is not a big name like Dylan Thomas or even Gill, but both Auden and Eliot unhesitatingly described him as a genius, and as you walk in the hills around Capel-y-ffin he is the one you most often find walking beside you. His drawings capture the spirit of the valley, and your experience of the place is ever afterwards affected by his interpretation. You can follow him and his friend René Hague up the hill behind the monastery to where on Christmas Eve 1924 they unblocked the stream when an unfriendly neighbour had cut of the water supply. You can walk over to the Bull at Craswall, as they would have done, three hard miles on foot, at least 15 by car - the public bar has hardly changed since their time - or over to the Olchon Valley, another secret place.
Opposite the monastery, you can climb to the top of Y Twmpa hill, possibly finding the waterfall where Gill's daughters bathed on hot summer days. From there, you can walk along the spur known as Lord Hereford's Knob then back down the road to the little church with its lopsided belfry surrounded by ancient yews. The churchyard has a gravestone engraved by Gill and two by his pupil Laurie Cribb. Whatever you may think of Gill as an artist - much of his drawing is by any standards bad and his typefaces are difficult to use well, elegant but never quite the right weight, and surprisingly hard to read - there is no doubt that he and Cribb had few equals when it came to carving letters in stone. In Capel-y-ffin churchyard, their work is instantly recognisable; they are master craftsmen among journeymen. It is rare these days to find major works of art that are not fenced off, and it is a rare privilege to be able to kneel down in the churchyard and run your fingers along the grooves in the stone as the rain runs down the back of your neck.
The church itself is tiny, only 25ft by 12, and very simple, designed for a congregation of no more than 20. At the far end looking out on to the mountains is a plain glass window etched with the words: "I shall lift up mine eyes to the hills whence cometh my salvation."
For a time after the Gill community left, the monastery became a school for girls aged five to 15; in the prospectus, Gill was listed as a visiting art teacher; it didn't last long. Then it became a guest house, again without success. In the 21st century, it appears to be in a state of suspended animation, always undergoing repair, rarely showing much sign of progress, never anyone there. Once I found a child's tricycle on end, not quite with the rear wheel still going round but eerily deserted, waiting for something to happen.
Where to stay
The Barn, Pen-y-Maes, Capel-y-ffin (01873 890477, thebarn-wales.co.uk) is a vegetarian B&B with yoga and drawing weekends, singles £25 per night, doubles £45, en suite £50, children £15. Gill's great-granddaughter offers pony trekking and accommodation at The Grange, next door to the monastery (01873 890215, grangetrekking.co.uk), £25pp B&B.