Tradition gives the founding to Charlemagne in 785 - the kind of pious attribution that medieval houses liked because emperors made better patron saints than the actual founders. The real date is probably 815, and the real founders were probably two men named Bozo and Bardo, who established a community of nuns or a women's collegiate foundation in the Dreingau region of Saxony, recently subdued by Frankish arms. Whatever it was at the start, by the 12th century the community had drifted enough that in 1131 Egbert, Bishop of Münster, expelled the women and replaced them with Benedictine monks. The new house took root.

From the 13th century onward, Liesborn followed the path so many German abbeys followed: it got rich, and ascetic life slid downhill. By 1298 the abbey's property had been divided into twenty-two full prebends for nobly born canons and another six for boys - effectively converting a monastery into a secular college for the local aristocracy. The 14th-century plague hit hard but the wealthy abbey survived. Then in 1465, in one of those decisive reform moments that the Bursfelde Congregation kept making across northern Germany, Liesborn signed on to the reform. Discipline returned. The monks started behaving like monks again. Heinrich of Cleves became abbot the same year, and held the position until 1490; his successor Johann Smalebecker held it from 1490 to 1522. Between them they rebuilt the church, fixed the finances, and made Liesborn an exporter of reform - other Benedictine houses and several nunneries adopted the new discipline under Liesborn's influence.

Abbot Heinrich also commissioned the altarpiece that would make his abbey famous five centuries later. Painted around 1465 by an anonymous Westphalian master, it was a vast multi-panel work centred on a Crucifixion and surrounded by scenes from the life of Christ, the Virgin, and various saints. We do not know the painter's name. We do not know where he trained, though stylistic analysis suggests Cologne. We know only the work itself - and that the work is extraordinarily good. The figures have a quiet, withdrawn dignity. The colour is jewel-like. The compositions have the patience of meditation. Art historians have spent more than a century trying to pin down his identity, with no success. Hence: the Master of Liesborn.

The good century did not last. Abbot Anton Kalthoff (1522-32) embraced Anabaptist doctrines and was deposed. Gerlach Westhof (1554-82) tilted Protestant and ran the monastery deep into debt. The Thirty Years' War and the wars of the 18th century ground the finances further down, so that by the time of the great secularisation of German church property in 1803 - dissolution on 2 May, by Prussian decree - Liesborn owed thousands of thalers and had little to pay with. The Prussian Crown took the buildings. Then in 1807, with the region briefly under French administration as part of the Napoleonic reshuffling of German states, the altarpiece was broken up and sold off cheaply. Panels scattered across Europe. By the time the dust of the 19th century settled, the finest of them had been acquired by the National Gallery in London - where they form one of the highlights of the Gallery's collection of early German painting. Other panels are in Münster, in private collections, and in museums from Berlin to Mannheim.

The Gothic church, rebuilt between 1499 and 1506 under Abbot Smalebecker's reform program, is still standing. Several of the monastic buildings are still standing. The village of Liesborn, now part of the municipality of Wadersloh in the district of Warendorf, has a small but earnest museum - the Museum Abtei Liesborn - which displays reproductions of the altarpiece panels alongside genuine medieval art from the region. The historian-monk Bernhard Witte, who lived here from 1490 to about 1534, wrote a history of Westphalia and a chronicle of the abbey while sitting in this exact place. The church bells still ring. The grass grows over fields that monks once worked. And the great paintings hang in Trafalgar Square, where they are described by gallery cards as the work of an unknown master from a place most London visitors have never heard of.