Richard Hammond and the Holy Grail, an hour-long BBC1 documentary, is the latest offering from the mini-industry spawned by The Da Vinci Code. The Holy Grail has long fascinated Christians, but the 30 million sales of Dan Brown's book have propelled the legend to stratospheric popularity. From spin-off books to TV and even £144 Da Vinci Code breaks from VisitScotland, everybody wants a slice of immortality.

BBC1 has deeper pockets than the average tourist, so it sent Hammond on a 5,000 mile odyssey, crossing four countries, to find out. The result is what you could term "occultism lite" - Hammond is known for his cheeky-chappy style, so the BBC clearly didn't intend this to be a furrowed-brow exercise in academia. Hammond begins his quest with: "The BBC - bless 'em - have given me just two weeks to search for the Holy Grail so, er, I'd better get on with it."

He sets out the basics of the Grail story by flying to Istanbul, or Constantinople as it was once known. The city is significant as a kind of geographical crossroads, regularly used by crusaders heading east to the Holy Land or returning to Europe.

Legend has it that the Grail, the cup drunk from by Jesus at the Last Supper before his crucifixion, was seen by pilgrims in the 5th century. However, it seemed to disappear from Palestine, apparently resurfacing in Constantinople in the 13th century. Constantinople was a clearing house for religious relics. At various times pieces of the one true cross, the crown of thorns, John the Baptist's bones and the lance that pierced the side of Christ are all supposed to have passed through there.

So is the Grail there? The conclusion of Tom Asbridge - an expert on the crusades - is "possibly". So many other relics appeared at points it is conceivable the grail was among them.

The returning crusaders are thought to have taken some relics with them as they scattered back to Spain, France and Britain, so Hammond - on the toss of a coin - heads to Britain. He takes an inconclusive trip to Temple Church in London, on the grounds that it was built by the order of the Knights Templar, who loom large in grail history. Set up to safeguard the progress of pilgrims to the Holy Land, the order grew rich and powerful - until it was violently suppressed in 1312 on the orders of Pope Clement V. Some say the order attracted the envy of monarchs because of its wealth and privilege; others contend the knights followed deviant occult beliefs.

Is the Grail in London? Who knows, says Hammond, and heads off to Scotland. He goes to Rosslyn Chapel in Roslin, near Penicuik, a must-see for Da Vinci Code fans. Built in 1446 by Sir William of St Clair, legend suggests the chapel may have been used by the Templars as a hiding place for religious relics - and, conceivably, the Grail.

Hammond describes the chapel as "a sort of temple to medieval bling". Could the Grail be there? Ian Robertson, author of Rosslyn and The Grail, says no: "The order of the temple was suppressed way before this [Rosslyn[ was built.

"This was built by a 15th-century knight, but not a Knight Templar. In the last 20 years stories of the Templars building Rosslyn have grown up, but they are based on no historical fact." The only place the grail exists in Rosslyn is, it seems, as an image in a stained-glass window.

Next stop is Glastonbury, on the assumption that the King Arthur stories may yield some clue. Did Joseph of Arimathea carry the grail back from the Middle East and bury it in Glastonbury? Cue the sceptical historian, Geoffrey Ashe, author of The Discovery of King Arthur: "In the stories it is magnificent, it is jewelled, it shines, it hovers in the air, it is a supernatural object altogether. It's quite mistaken to equate it with a thing you could actually find and put in a museum or anything like that."

The connection between the Grail and Glastonbury is evidently derived from the 19th-century poetry of Alfred, Lord Tennyson. Tennyson in turn took it from French romances of the medieval period, which originated with the work of French writer Chrétien De Troyes in the 12th century. And Chrétien, Hammond says, "made the whole thing up".

The Da Vinci Code, however, revolves around the premise that the Grail is not a physical object but a secret - the fact that Jesus had a relationship with Mary Magdalene and children, of his bloodline, were born as a result - and that this secret is guarded by a mysterious organisation called The Priory of Sion, whose members reportedly included Leonardo da Vinci and Victor Hugo.

This theory leads Hammond to some rather fetching photo opportunities in France's Cathar country - the medieval towns of Carcassonne and Montségur in the Pyrénées. One theory goes that the Cathars, a heretical sect, knew Jesus was a man, not divine, and that they had his skeleton. Mary Magdalene was supposed to have sailed with his body to the south of France.

While in France, Hammond tries to nail another story of the Grail myth: that an impoverished priest, Abbé Bérenger Saunière, stumbled across a Grail-related secret while renovating his church, leading him to unimaginable wealth. Again, all is speculation. The only people likely to be enjoying unimaginable wealth here are owners of the local hostels, where three million tourists arrive every year.

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