St Thomas Becket was less lucky. Henry had already ordered that the cult of this rebellious prelate be utterly expunged. The numerous altars and images in his honour in churches across the country were destroyed and his shrine at Canterbury smashed.  The location of his bones remains a mystery. A near-contemporary account claims that Henry's men burnt them, so dangerous was the martyr's cult considered to be. Today, of the English saints, only St Edward the Confessor still lies in his medieval tomb. Henry had him reburied but the tomb survived, perhaps due to an oversight. The Catholic Mary I replaced the royal corpse within it.

The devastation of the Reformation did not end with the shrines of saints. In the reign of Edward VI almost all remaining religious art in the country was lost. A few wall paintings and painted screens survive, but of the images of Christ crucified that were mounted in the chancel of every parish church, not a single one remains. The English destroyed almost their entire artistic tradition and then suppressed their memory of it. Today, medieval alabaster sculptures hold places of honour in museums across Europe and North America, but few of us recognise them as products of the Midlands.  A 15th-century example portraying St Thomas Becket's murder is included in the British Museum's show together with various Canterbury pilgrims' accoutrements. 

The Treasures of Heaven exhibition is devoted to reliquaries, the bejewelled cases fashioned to hold relics within. The expense and time devoted to crafting these holy boxes gives some indication of how seriously relics were taken. Visitors expecting to see gruesome remains will be disappointed — there is hardly a bone in sight. Most of the relics are discreetly wrapped in cloth and, in any case, they are very small in comparison to the reliquaries built to house them. Instead of grisly bits of cadavers, we are treated to a very fine exhibition of medieval objets d'art. 

Some of the artefacts on display are stunningly beautiful and shown to maximum advantage. Many are mounted in well-lit freestanding cases rather than against the walls. This means, for example, that we can admire the intricate gold beadwork on all four sides of the 1,000-year-old portable altar of Countess Gertrude. Although some of the pieces, like the Franks Casket, will be familiar to devotees of the British Museum, there are plenty of treasures that have never before been seen in the UK. Magnified photographs and film show the microscopic detail on some of the smaller exhibits and allow us to see them both open and closed.

Most of the pilgrims flocking to the shrines of saints would never have got as close to the relics as we can today. But that did not mean they thought the trip was wasted. Their sacred journeys could reduce the time a soul had to spend in Purgatory after death. It was not fear of hellfire that motivated late medieval piety. Hell was for infidels and heretics, its horrors sending a righteous shiver down the spines of congregations as they admired the last judgment or "doom" painted on the walls of most churches. Purgatory, on the other hand, was a clear and present danger to almost all Christians who knew that they could not ascend straight to heaven. They had to be cleansed of their sins through a posthumous process that was at the least unpleasant and quite possibly very painful and drawn out. While the souls in Purgatory knew their salvation was guaranteed, it was well worth minimising how long they had to stay there. 

Of course, many invalids prayed to saints for miraculous healing. There was nothing irrational about this. Until the 19th century, professional physicians could do almost nothing for their patients. Treatments such as bleeding, emetics and purgatives were likely to do more harm than good. Supplications before a relic might have no physical effect at all, but neither would prayers make an illness worse. And for the faithful, what we now know as the placebo effect could very well lead to cures more impressive than anything contemporary doctors could manage.