The truth is . . it's a code of old rubbish

Because the number of such visitors is about to explode in the wake of a book that manages to place the postcode EH25 9PU at the epicentre of a 1000-year-plus global power struggle.

For those of you who have been living in a hole in the ground under a medieval church for the past three years the book - The Da Vinci Code - is a thriller by Dan Brown that has sold 35 million copies worldwide and will shortly be released as a film starring Tom Hanks and Sir Ian McKellen.

The fantastical plot revolves around a murder in the Louvre and the existence of an across-the-centuries conspiracy to stop "the truth" getting out. And obviously if you want a believable plot about where "the truth" would be hidden then you would naturally think of somewhere in Midlothian, as anyone who has ever had to rely on the local bus services can attest to (particularly after 7pm which is when Evil Forces knock off). Given how difficult it is to get to Tesco you can imagine how much more difficult getting to "the truth" might be.

Strangely enough it turns out that there is a good historical reason to use Midlothian as the setting for sinister doings other than the transport infrastructure and age-old reputation.

Rosslyn Chapel, a 15th century chapel built by William Sinclair, is a pick-and-mix of medieval symbols, much like you would expect a church to end up looking like if you marooned a bunch of stonemasons in a town without any leisure facilities for too long. In focusing on these stone doodles the new historians have arrived at the idea that the Holy Grail or something like it is inside Rosslyn Chapel and protected by dedicated guardians.

This is almost right. The dedicated guardians are from Rosslyn Chapel Trust and are employed to stop the half-wit battalions trying to lever up the ancient church flagstones using their divine instruments from the nearby B&Q.

As for the Holy Grail bit, in those days EVERYONE said they had the Holy Grail. It was part of that wonderful religious crusading stuff where people went around saying My God is Bigger Than Your God and to prove it I've got the Holy Grail. Where? I've hidden it. Where? Somewhere you'll never dare to find it. Oh, Midlothian.

To whet the appetite that bit more all you need to do is throw in a few spooky sounding organisations like the Knights Templar and Opus Dei and you have all the necessary fog and lights for a proper now-you-see-it-now-you-don't conspiracy based on almost no testable facts whatsoever (For those who don't know, Opus Dei is an elite group inside the Roman Catholic Church with exclusive Members Only access to all manner of privileged information and late-night opening; sort of the Marks & Spencer Account Holders of the Vatican).

Dan Brown's book - based on what just might have been historically possible - has quite definitely created an army of the happily gullible, people for whom that whole JFK thing has never been properly explained and ask have you ever noticed how the shadows are all in the wrong places in those Apollo 11 moon photographs with Neil and Buzz?

An almighty marketing campaign will ensure at the very least that this army of the happily gullible will multiply a thousand fold, yea even as rabbits in the field. Before long the cobbled streets of Edinburgh will overfloweth with big-boned couples equipped with guidebooks and a desperate need to find "Roz-Line". (Here is a tip for fun: Don't tell visitors how far it is to Rosslyn Chapel. Instead point south-east and say: "It's just a wee walk that way" and enjoy the scene as the ant-like column of soggy tourists try to nip over the city bypass.)

For those of you who haven't read the book but particularly for those of you who have, I am happy to reveal "the truth". There is no secret. Nothing has happened. Nothing will happen. Get back to work.

Wait, I hear some cry, everyone knows there is no smoke without fire. Wrong. There is no smoke without idiots who can't tell that it is not smoke but in fact the mists of time. As you know the mists of time make everything appear possible, including the possibility that a small village near Dalkeith conceals a truth so profound only the man who managed to save Private Ryan can find it.

In the world of conspiracy theory, facts are stranger than fiction

CONSPIRACY theories like the one behind the Da Vinci Code appear when people are at a loss to explain things, and/or feel in some way powerless.

Generally, the theory provided will either be unprovable and therefore "right", or else be incredibly appealing because it links a perceived problem with a group of people no-one likes - asylum seekers are stealing all our shopping trolleys, for example.

What is striking about the Da Vinci conspiracy is how it manages to divert attention away from the real truth about Rosslyn. Think about it. How plausible is it that one wee town could be home to both the Holy Grail and the world's first cloned sheep? While Dolly now lives at the Royal Museum of Scotland, looking slightly less animated than those TV pictures suggested, her origins at the Rosslyn Institute are not in doubt. But why would a research institute open up so close to this little church?

The truth is, the Holy Grail myth has been invented to cover up what is really beneath the ground - a vast bunker filled with identical sheep.

Every now and then, one pops out and watches as you go to Ikea.