Ray Godson and Stephen Coates are both keen to get their hands on a key to the thick bronze door that shields the entrance to an imposing granite mausoleum in Brompton Cemetery in west London.

For Mr Godson, a replacement for the key that went missing in the late 1970s would allow him to pay his respects to Hannah Courtoy, his great-great-grandmother, who is interred in the tomb with two of her three daughters. Mr Coates’s interest is rather more esoteric: he is keen to test a long-held theory that the tomb is a fully functioning Victorian time machine.

If the door is opened, Mr Godson expects to find “a few dead birds and a lot of dust”. Mr Coates hopes to be whisked back to ancient Egypt or – and this is his preferred option – to emerge from the tomb as a younger version of himself.

“I’ll certainly give it a go,” says Mr Coates, a film composer with a long-standing interest in London and its myths. “Who wouldn’t take the opportunity to travel through time?”

Mr Coates is so keen to unlock the secrets of the Courtoy mausoleum – or the Courtoy Time Machine as it has been dubbed – that he has organised a storytelling event in the cemetery on 20 December to raise enough money to buy a new key. As he points out: “You can’t just go down to the high street and get a Yale key cut.” What is required is “a big, copper, Lord of the Rings job”. And it will need to be made by a heritage locksmith, because the mausoleum is a listed monument.

The idea that Hannah Courtoy’s tomb is a time machine – a kind of stone-walled Tardis – has been fermenting for years. It is based on a volatile mixture of historical fact, supposition, and unfettered flights of the imagination.

It is true that Hannah, who died in 1849, was most likely acquainted with Joseph Bonomi, a well-known sculptor and Egyptologist whose relatively modest gravestone lies only a few metres away from the Courtoy mausoleum. It is rumoured that Bonomi designed Hannah’s mausoleum, a theory given some credence by its pyramid-shaped peak and by the mysterious hieroglyphics inscribed on the walls of the tomb and on Bonomi’s own headstone.

The Victorians were fascinated with the idea of time travel, and some believed the pharaohs had discovered its secrets. Might Bonomi have learned the time-travel trick during one of his expeditions to the pyramids?

If he did, he could have passed it on to his business partner, a mysterious London inventor of naval weaponry called Samuel Alfred Warner, who is also buried in Brompton Cemetery. Warner’s inventions included an invisible shell, or, as Mr Coates puts it, “a bomb that could be teleported a short distance – a kind of psychic torpedo”.

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Incredibly, the Royal Navy was so intrigued by Warner’s metaphysical armament that it allowed him to stage several demonstrations. A least one ship was destroyed during these trials, although none of them offered conclusive proof of the weapon’s efficacy.

The connection with Warner and his “psychic torpedo” has convinced Mr Coates that the Courtoy mausoleum is more likely to be a teleportation chamber than a straightforward time machine. He posits that it was part of a network of chambers erected in the “magnificent seven” cemeteries that were built in a ring around central London in the 19th century. An eighth mausoleum resembling the Courtoy tomb is located in Montmartre Cemetery in Paris, so if the teleportation theory is correct, the network might be used to pop over the Channel as well as whiz around London in split seconds, and without paying the congestion charge.

The time machine myth received a significant boost in 2003 when the Scottish musician Drew Mulholland – who records under the name Mount Vernon Astral Temple – put a photograph of the Courtoy mausoleum on the cover of his album Musick That Destroys Itself. The doctored image shows an eerie vortex emanating from the doorway of the tomb.

“I got fascinated by the idea [of the time machine],” said Mulholland, who is currently a composer in residence at Glasgow University. “The notion that it develops its own energy or current ….”

Supporters of the time machine theory often claim it is significant that the Courtoy mausoleum is the only tomb in Brompton Cemetery for which there are no plans and no key. Not true, says Robert Stephenson, a cemetery guide whose passionate expertise has earned him the nickname “Dr Death”. There are no plans for several other prominent tombs in the cemetery and although the key to the Courtoy mausoleum is missing, the popular idea that it has not been opened since Hannah was laid to rest is incorrect.

The time machine theory is a “lovely idea and it’s brought a lot of interest in the cemetery”, said Mr Stephenson. “But I wouldn’t say I was totally behind it.”

Mr Godson is equally sceptical. “It would be lovely if there was some substance to it,” he said, “but I’m doubtful.”

Mr Coates isn’t put off by the non-believers.

“I know he’s [Mr Godson] not a subscriber to the time machine theory,” he says, “but that in itself is suspicious in my view.”

Ultimately, the mystery won’t be solved until a new key is inserted in the lock and the heavy bronze door swings open to reveal the tomb’s secrets.