“So,” says Jan Sadleir with military briskness. “You want to do The Knowledge?” The first step, she says, is an interview with the Public Carriage Office of the Metropolitan Police to assess suitability. She looks me over and dispenses advice.

“Get your hair cut, get your nails trimmed, and wear a collar and tie. Make sure you’re not wearing any jewellery apart from a wedding ring. No earrings. And you must make sure that you’re never, ever late.”

This is not an Army recruitment office. This is where drivers come to learn The Knowledge - the mental A-Z that allows a cabbie to find his way effortlessly across London’s maze of streets.

Jan Sadleir is a taxi-driver and principal of KPM UK Taxis Knowledge School in Whitechapel, one of a handful of training places for London’s cabbies. She says this training is a long, gruelling - but ultimately rewarding journey.

Her own grasp of The Knowledge is extraordinary. “If I were to close my eyes, I could see a map of London in my head. And on a good few streets I could see buildings, pedestrian crossings, parks. I can visualise a street just as well as if I were looking at a map.”

Taxi-drivers learn to do this with the help of the Blue Book, officially entitled The Public Carriage Office List of Questions. And the first question students face is how to get from Manor House Station to Gibson Square? This you must learn by rote.

Barely pausing for breath, Mrs Sadleir demonstrates: “Leave on the left Green Lanes, bear right Petherton Road, cross Grosvenor Avenue, forward Wallace Road, cross St Paul’s Road, forward Canonbury Park North, left Grange Grove, right Canonbury Place, forward Canonbury Square, forward Canonbury Lane, left Upper Street. Right Barnsbury Street, left Milner Square, forward Milner place, Gibson Square is facing you.”

There are 400 routes like this in the Blue Book. And that’s just central London. Another surprising thing is that trainee cabbies don’t even look at a taxi until they’re about two years down the road. Most beginners get to know the streets of London on a moped.

“You can do it on foot, or on a bike - but the recommended way is by moped. You’ve got to be able to stop here there and everywhere, get off, have a look at buildings. It’s a very simple way to get around town.”

Inevitably, this business has its own language - rooted in tradition. Runs that go virtually as the crow flies are called “cotton” routes. If a cabbie starts talking about “oranges and lemons” he means main roads, not fruit. Someone who has finally gained his badge is called a “butterboy”.

But before you get there, there are the exams. After two years of reciting routes and chugging around on a moped, you sit a series of written and oral tests at the Public Carriage Office.

The oral exams - called “appearances” - are very tough, says Jan Sadleir. You could be asked to recite your way street-by-street from one London landmark to another, and the examiner just might decide to tap his pencil impatiently while you’re doing it.

In another part of the exam, students get a blank map of a London area and are asked to put in roads and places of interest. There is also a driving test in a taxi, and then they finally learn the suburban routes.

Most drop out - Mrs Sadleir estimates around 80 per cent. But she says those who gain their badges then have a secure job for life. “I’m very, very proud of every student who gets their badge. I think they have done an absolutely wonderful thing.”