This is a very short novel, only just over a hundred pages: its story is told more by allusion and gradual layering than by the narration of events. I'm still unsure whether it is understated or willfully obscure.
Vane, an agent of the Church, arrives on a remote Welsh island with his servant Geoffrey and his horse Saviour. He's been sent by the Bishop to investigate the mysterious disappearance of many pilgrims who have visited the island's sacred well. The Abbot is not exactly helpful: the island community is oddly imbalanced, consisting mostly of young novices with few (if any) older monks, all supported by a group of young women overseen by Mrs Ffedderbompau. (This is the sort of name that, having read far too much humorous fiction, I keep expecting to contain some joke in its pronunciation.)
Vane's investigations lead him to the oddly empty burial ground at the centre of the island, and to the holy well itself, beneath the abbey. Meanwhile, the abbot is wandering the unknown passageways of his own house, investigating life and death, and musing on the possibility of resurrection: meanwhile, Geoffrey is falling in love with one of the maids; meanwhile, Mrs F is dying, and Saviour is transformed. And by the end of the book it seems that some irreversible, miraculous, grotesque change is underway, though it's not entirely clear what has triggered it.
The language is beautiful and dense -- Fuller's primarily a poet, though this novel won the Whitbread Prize in 1983. There are some gruesomely precise images, and a sharp, visceral vividness to each incident, each scene. It's not clear when all this is happening: it could be any time between 1106 (there's a reference to something published that year) and the present day, though I'd guess at some time between 1400 and 1750.
The Abbot ... feared the process of animation induced by the miraculous spring, feared the active weight and muscle of the words and bindings of his once-still books. It must be the curing of the leather that rendered it capable of returning to its former shapes. ... He stood with his hand on the knotted bark of the library door in despair as it thudded against his palm with the weight of the huddled herds inside.