“I’ll just open the hatch…” says Richard Smith, who is stooped over in the ticket office at the Monument. He’s examining the oak-panelled floor as though it’s hiding a secret chamber, as in an Indiana Jones movie. Above him, a desk is piled high with leaflets “This is to certify that ________ has climbed the 311 steps of the Monument”.

The thing is, there are actually 345.

The Monument to the Great Fire of London consists of a towering, 202-foot (61-metre) stone column, decorated with dragons and topped with a flaming golden orb. On the inside, a striking spiral staircase stretches all the way to the top, twisting up like the peel of an apple carved in a single, continuous ribbon.

For years, a cracked plaque tacked to the base explained that it had been designed by Sir Christopher Wren.

Again, this isn’t entirely truthful.

It makes a lot of sense that the capital’s beloved Monument would be borne of Britain’s most celebrated architect. After all, Wren was widely involved in rebuilding London after the Great Fire destroyed pretty much every inch of ground within the city walls – including 13,200 houses and numerous extraordinary public buildings, from riverside castles to Whittington’s Longhouse, one of the largest public toilets in Europe. He even created the nearby St Paul’s Cathedral.

In fact, the Monument was designed by his close friend: a scientist.