Tom McRae has become used to tourists nervously interrupting him as he goes about his work. The 50-year-old gardener at Rosslyn chapel listens politely as they ask their arcane questions. Where exactly are the ley lines? Does he know the hidden meaning of this or that carving? Would he be kind enough to point them in the direction of the Holy Grail?

"I don't know what they ask me for," he sighs, leaning on his lawnmower and running a hand over his balding head. "There's been so much written about the place, and there's so many clues inside to what it's all about. I think they expect a bit too much from me. Some of their theories are a bit way out, but if they want to believe something, let them."

Rosslyn, tucked into lush Midlothian countryside seven miles from Edinburgh, has beguiled visitors for five centuries. In its famous stonework are carved pagan, Christian, masonic and Arabic symbols, offering countless clues to various theories about the place.

Tales of the mysterious Knights Templar provide another layer of conjecture. Add in some New Age mysticism and a sprinkling of wackos and you have a heady mixture. The result is that Rosslyn has become Scotland's grassy knoll, irresistible to the world's conspiracy theorists and fantasists. Perhaps, inevitably, America in particular is lapping it up.

Fuelled with the passions Rosslyn seems to incite, dozens of books have been written about the place. Depending on your taste, the 15th-century building is the secret resting place of the mummified head of Christ, a collection of lost biblical scrolls, the ark of the covenant, the real stone of destiny, Herod's gold, a piece of the true cross or, most famously, the Holy Grail, the cup used by Christ at the last supper. Recently the grounds were surveyed by Strange Phenomena Investigations, a group of UFO enthusiasts from nearby Denny, who concluded the chapel was an astral doorway that could transport people into different galaxies and dimensions.

Rosslyn's allure may or may not be astral, but it is certainly going global. Many of this year's visiting tourists, especially the Americans, carry well-thumbed copies of one particular book. The Da Vinci Code has been a global bestseller for author Dan Brown, selling 7m copies in America and more than 500,000 in Britain since it was published last year.

Visitor numbers at the chapel are almost 40% higher than last year, an increase the Rosslyn Trust believes is largely down to the book. The numbers will undoubtedly swell even more next year with the release of Columbia Pictures' film of the book. The team behind the Oscar-winning A Beautiful Mind — director Ron Howard and scriptwriter Akiva Goldsman — are signed up and according to Variety magazine the frontrunners for the leading role are Tom Hanks, George Clooney and Russell Crowe. Rosslyn is about to become a global phenomenon.

The book's fictional central character is Robert Langdon, a Harvard professor, who is asked to help solve the murder of a curator at the Louvre in Paris. This entails breaking a series of baffling codes, unearthing secrets about historical figures such as Isaac Newton and Leonardo da Vinci, and confronting a secret held for two millenniums by dark forces within the Catholic church. Its dramatic denouement takes place at Rosslyn, although it would be a shame to divulge it.

The fanatical interest of the tourists is only heightened by their visit. From the outside, the impact of the chapel's gargoyles and flying buttresses is spoiled by a massive steel structure protecting the roof during renovation work. But inside the effect is breathtaking.

Almost every square foot is covered with intricate sculpture and carvings — foliage, symbols, animals and figures both human and divine. The vaulted ceiling is decorated with stars, daisies, lilies and roses. Every pillar and window has its supporting cast of angels carrying hearts, scrolls, swords and crosses. One is playing the bagpipes.

The effect, it has been pointed out, is more like cake icing or topiary than stone. You peer at what looks like a mass of intricate decoration and suddenly a face comes into focus — sometimes pious, sometimes demonic. Over there is a depiction of the seven deadly sins, here the dance of death. Is that a monkey? Is that a lamb? There are more than 100 depictions of the Green Man, a pagan fertility symbol.

In this setting, The Da Vinci Code is either an exhilaratingly brainy thriller or the shaggiest of shaggy dog stories. The son of a maths professor with a lifelong interest in code breaking and secret societies, Brown likes a conundrum and clearly enjoys having an obsessive fan base. His website offers them toothsome titbits about himself, such as his use of "gravity boots" fixed to his ceiling in between bouts of writing. "Hanging upside down seems to help me solve plot challenges by shifting my entire perspective," he confides.

As part of his research, Brown spent time in Midlothian and questioned officials of the trust at length. "For centuries this stone chapel had echoed with whispers of the Holy Grail's presence," reads one passage. "The whispers had turned to shouts in recent decades when ground-penetrating radar revealed the presence of an astonishing structure beneath the chapel — a massive subterranean chamber. Not only did this deep vault dwarf the chapel but also it appeared to have no entrance or exit.

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