The accountancy firm that looks after children's entertainers the Wiggles is not an obvious place to search for the Holy Grail, but that's where the trail led last night. It started with a simple quest - what on earth is a large advertisment headlined "The Ancient & Noble Order of The Knights Templar" doing in the Daily Telegraph? - and it led your intrepid investigator to the wilds of west London and then all the way back to the 12th century.

It was around 1118 when the order of the Knights Templar was founded in the Holy Land by Hughes de Payens and eight other French knights to protect pilgrims and defend Jerusalem, which had been captured by the Crusaders in 1099. Over almost two centuries, the order grew into one of the most rich and powerful institutions of the era. It all came crashing down when the Pope burnt the Templars' last grand master at the stake in Paris in 1314. The order seemed to have disappeared - until yesterday, when this tantalising advertisement appeared.

Apart from the odd misplaced apostrophe and various arcane references to "annulling the bull", the advert gravely announced that the Knights Templar would petition the Pope to "restore the Order with the duties, rights and privileges appropriate to the 21st century and beyond". It called on all Templar groups and "brothers in arms" around the world to get in touch, either via its website, www.theknightstemplar.info, or an address in west London, which could clearly become a mecca for long-lost Templars and baffled Telegraph readers alike.

My quest was to decipher this advertisement and find out why someone would pay several thousand pounds to place it in the press.

Historians agree that the Knights Templar were a powerful military order of warrior monks charged with defending the Holy Land. They amassed great wealth, although their prestige was damaged when the Christians were driven out of Jerusalem in the 13th century. King Philip IV "the Fair" of France is also said to have become indebted to the Templars, and in 1307 ordered the arrest of the grand master of the order, Jacques de Molay, and other key leaders, who were hit with the usual concoction of lurid charges: heresy, sodomy and devil worship. Philip persuaded his pal Pope Clement V to issue a bull suppressing the Templars in 1312. Two years later, Molay was burned at the stake on an island in the Seine.

That, you might think, was that: the cruel destruction of a religious order in the Middle Ages. But this particular nasty, brutish and short episode in medieval history has spawned long centuries of rumour about the Templars. "They have become a kind of mosaic of mythology and conspiracy, secrecy and survival and anti-papacy right across Europe and into North America," says Martin Palmer, a theologian and religious historian.

A popular myth is that the Templars lived on - in Scotland and secret rooms in Paris - hoarding their riches (one cup, used by Christ) and guarding their secrets (Jesus had a child with Mary Magdalene). And since the 19th century, all kinds of new Templar groups, often loosely connected to the Masons or taking on familiar Christian rituals, have popped up, claiming to recapture the spirit of the original order.

Last year, the Vatican felt so besieged by the weight of conspiracy and conjecture drummed up by Dan Brown's Da Vinci Code that it took an unsual step: it released a document from its archives called the Chinon parchment, which shows that Pope Clement V's investigation into the Templars in 1308 actually cleared them of the heresy charges that led to many being put to death. Members of Templar groups have called on Pope Benedict to make a formal apology; the Vatican instead issued a collector's edition of the documents for £4,000 a pop.

Why did Rome take such a step? "The Catholic church has been shaken by The Da Vinci Code et al," says Palmer. "It was trying to put its side of the story and quash the idea that the Knights were Satanists conspiring to overturn Christendom. It's a full-frontal assault on the conspiracy theorists, and that's why the document was published with considerable fanfare."

What the advert demands is a formal restoration of the Knights Templar. Such a move would not be without precedent: the Jesuits were suppressed by the Catholic church in 1773 before being formally restored by Rome in 1814. "In the background is a mix of Da Vinci Code nonsense and a hard-headed real estate issue," says Palmer. "The consequences of the papacy restoring the Knights would be to open up an enormous can of worms."

One of the worms is property. While Palmer dismisses contemporary "orders" that claim lineage back to 14th-century Templars, he believes modern Templars could in theory claim rights to property seized by the Catholic church. There could, for instance, be a dispute over the ownership of the Temple Church in London (which passed to the Church of England).

Another possible worm is the reaction that the restoration of the Templars would cause among some Islamic groups who associate the Templars with the Crusades. Palmer believes the Pope, who has worked hard to build bridges between Islam and Christianity after several gaffes, would be wary of triggering more conflict with the Muslim world.

And so I find myself in west London, at the address given in the advert. It turns out to be the offices of Sloane & Co, a small accountancy firm set up by David Sloane in 1974. Sloane is no boring bean-counter - he's a rock accountant. Over the years, he's done the books for INXS, Maxi Priest, Mark Morrison and even Melinda Messenger. His top clients today are the Wiggles, the Australian preschool entertainers who sport primary-coloured turtlenecks and have made millions from selling CDs of songs about Dorothy the Dinosaur.

The connection between turtleneck-wearing entertainers and a secretive group of warrior monks has me stumped. At Sloane & Co's offices, the stairs are being redecorated. Hang on: is that an image of the Turin Shroud in undercoat? Only kidding.

According to the Charities Commission, Sloane is the official representative of the Knights Templar Trust charity, which is listed on the advertisement. Sadly, Sloane is not at his office and does not return my calls. All he will say is the Ancient & Noble Order of the Knights Templar "is a client of Sloane & Co". Why does the client want to reinvigorate and restore the Knights Templar? Apparently a press release in a couple of days will reveal all.

The Templars' website is registered under the name of another accountant. Are the Templars a group of accountants? Sadly, this man is said to have left the company of tax accountants on the Isle of Man three years ago and he eludes my attempts to find him. Still, if the Templars hid from us for 700 years through a combination of secret trap doors and shy accountants, it's probably no disgrace that I can't locate them in one afternoon.

"Were there a serious attempt to re-establish the order, we would see all kinds of funny creatures coming out from under the stones," predicts Palmer. "This could be an interesting meeting of mysticism and Mammon."