It is literally impossible to give a complete picture of art in England over the century and a half following the Norman Conquest - too much has been lost through vandalism, disdain, and time. The organizers of the magnificent and beautifully staged exhibition at the Hayward Gallery here (through July 22 - although there are very broad hints of an extension beyond that date) are the first to point this out. But in terms of what is extant, this display of splendid items and fragments of ''English Romanesque Art'' could hardly be bettered.
The show is a vast array of period work - including everything from carved stone capitals and figures to wonderfully sensitive or fanciful ivories, from chess pieces to liturgical combs, and from a truly astounding gathering of manuscripts to all sorts of metalwork.
The period covered, approximately 1066 to 1200, is what used to be known in architectural books as ''Norman.'' The royal luminaries of these years include William I (the Conqueror), Henry I, and Henry II (the ''Lion in Winter'' Henry).
As the surprisingly various styles seen in the exhibition indicate, the techniques and concepts introduced to England by the Normans, although dominant, were not the only threads woven into the fabric of the English Romanesque.
There were skills and traits recognizable as the vision of single, and in some cases still namable, artist-craftsmen. Master Hugo of Bury St. Edmunds, for instance, seems to have been a kind of proto-Renaissance-artist of this proto-Renaissance 12th century. His abilities extended to several of the varied art forms of the time: Bible painting, metalwork, and woodcarving.
Another influence the visitor keeps encountering is Byzantine art. Patrons, and also sometimes craftsmen, traveled abroad; in an age of vigorous and even competitive building, they were not slow to grasp fresh and unfamiliar modes of design.
This burst of ambitious building activity across England after 1066 was directly linked with the Normans' feudal form of government, and with their deliberate reform of religion. Churches and castles are therefore the main large-scale monuments of the Romanesque in England. This is one of the main, inevitable, limits in the exhibition: It is not too easy to move Durham Cathedral, or St. John's Chapel in the Tower of London, or Fountains Abbey Church in Yorkshire inside an art gallery, even the concrete bastion of the Hayward Gallery. An attempt is made with an audiovisual presentation, although it can be little more than a shadow of their full glory. With the array of marvelous objects on every side, however, it does prompt one to visualize something of the elaborate, and originally very colorful, grandeur of these massive architectural monuments. And it makes you want to go and see them. This is precisely an aim of the exhibition, of course, and one that is further encouraged by the section ''Rediscovery of the Romanesque'' in the 17th, 18th, and 19th centuries. Here can be seen the growing response of artists to the picturesque qualities of medieval architecture and ornament, the Romanesque in particular.
In presenting this exhibition, little has been spared of scholarly analysis. There is an excellent small guide, and an enormous catalog of exhaustive thoroughness. If more is wanted than the satisfying enjoyment of individual items, then the essays and detailed notes in the catalog are highly illuminating. An impressive understanding of the period's art is built from all these fragments.
Nowhere is the fragmentary nature of what remains more evident than in the stone sculpture. And yet it is here that the exhibition brings the visitor closest to the basically architectural context of the English Romanesque. These battered, broken blocks of stone - carved vividly with little monsters tied up in twisting foliage, with saintly heads or figures and with energetic and enjoyable decoration, often complex - somehow still manage to retain the mixed strength and fantasy, the mass and the embellishment, that is typical of the churches of the time.
Some of the finest stone items come from the later years of the 12th century - and have very recently come to light again: A number of excellent figural fragments were rediscovered in St. Albans in 1978, and some reliefs from a screen at Canterbury emerged during a recent restoration of the cloister, where they had been reused as mere building blocks.
The point is that the opulent and even extravagant imaginativeness of English Romanesque ecclesiastical art has not only been damaged by centuries of neglect and by the inroads of changing fashions and tastes, but was also subjected to wholesale destruction by the iconoclasts and vandals rife during the monastery-sacking time of Henry VIII and by the puritans of Cromwell's Commonwealth in the 17th century.