But the huge red jewel in the Imperial State Crown is the “Black Prince’s Ruby” worn by Henry V on his helmet at the Battle of Agincourt. So instead Taw Phaya Galae posits that the Nga Mauk ruby was used to decorate the Imperial Crown of India – a special crown made in 1911 for George V to wear in Delhi, when he and Queen Mary were proclaimed Emperor and Empress.

An internet search for “Nga Mauk ruby” tends to bring up multiple images of a ruby-studded Queen Elizabeth II and of the Imperial State Crown adorned with an enormous red stone - which suggests the Red Prince isn’t alone in making this supposition.

“However hard the British Imperialists tried to erase the trail of the Nga-Mauk ruby, there is compelling evidence of its existence,” he writes in The Nga Mauk Ruby in London. His theory is that the Nga Mauk may be hiding in plain sight – embedded in the Crown Jewels of Britain.

After writing numerous letters to the Queen and Prince Philip, and asking friends who visited London to check whether Sladen’s descendants were suspiciously well off, Taw Phaya Galae wrote a book setting out his theories about the fate of the ruby.

It’s thanks to this man - a fascinating figure known as the Red Prince on account of his years as a committed Communist - that Soe Win and I make a trip to the Tower of London, to inspect the Crown Jewels.

After Thibaw’s death, his youngest daughter, Soe Win’s grandmother, would take up the search for the Nga Mauk ruby, even writing a letter to the League of Nations. And when she died, Soe Win’s uncle, Taw Phaya Galae, took up the baton.

Although he gives no evidence for the hunch, the timing would work. An enormous spare ruby knocking around since the 1880s in the Royal Collection could theoretically have been donated for a new crown.

There is an obvious flaw in this theory: there is no ruby the size of the Nga Mauk in the Imperial Crown of India. The man who translated Taw Phaya Galae’s book into English has an answer, though. It was chopped into four, he suggests, and the parts became the centrepiece of each of the crown’s four sides.

We do know that the rubies in the crown are “of Burmese origin”, because this is stated in an official two-volume history of the Crown Jewels published in 1998. But that in itself proves little, as most of the world’s rubies come from Burma.

All the Royal Collection Trust will say is that “There is no archival documentation to indicate the origin of the rubies in the Imperial Crown of India.” They decline a request for an interview.

As Soe Win and I stand outside the battlements of the Tower of London, squat and brooding on the banks of the Thames, we cannot help wondering whether the Royal Collection is hiding something.

“I really believe it might be in here,” Soe Win confides and marches off through the portcullis gates. I have to run to catch him up.

I haven’t been to see the Crown Jewels since I was a child, and I now experience a curious mix of feelings, walking through this display of the power, pomp and prestige of the British monarchy, alongside a man who could have been a monarch himself.

Stepping on to the conveyor belt, we act like two excitable school-children, although with a combined age of 100. The regalia drift past in a sparkling blur of red, gold and silver. But we’re only interested in one piece.

When we reach the Imperial Crown of India, we linger as long as we can without causing a pile-up, and then rush round for a second go. Four rubies, each about 1.5cm in diameter twinkle out through the glass – could this be it? I stand staring so long that I lose track of Soe Win.