I can really only echo the sentiments of the other five-star reviews here. This is simply a beautiful work; another gem from Gollancz' Masterworks series, although one which reads more like fantasy or historical fiction than SF.
The novel is told through a series of six 'Measures', vignettes of story and mood focusing on a different character each time. While each works separately, taken together they form a tapestry linking thematic and narrative concerns - producing, ultimately, a beautifully-conceived and wonderfully effective tale of twentieth century England stifled by an all-powerful, anti-progress Catholic Church.
The alternate England is a triumph of understated, economical world-building (something that many of today's fantasy novels could learn from, perhaps). It is filled with enduring images - the Signallers' towers, the steam engines, the land held in winter's icy grasp - made all the more striking and memorable because we are shown them through the eyes of convincing and distinctive characters.
My only criticism would be of the 'Coda', which feels superfluous, and far too neat. Otherwise, this is a moving story of a transforming world, all the more effective for being incompletely explained.