“You get a sheet and you sign your name a few times, and then put a cross next to the one that you want to use,” says Sarah John, whose signature is about to go on a new £20 note.

“I won’t tell you exactly how many practices I had, but it was quite a lot before I found one that I was absolutely happy with. I wanted to make it so that you could read my signature. I thought, ‘If I’m going to have my name on there, people might as well actually know.’”

A new £20 note is being issued on 20 February as part of never-ending efforts to beat counterfeiters and, as chief cashier of the Bank of England (BoE), it is John’s signature that will be printed on it as an assurance of its value.

Bearing a 1799 self-portrait of artist JMW Turner and his 1818 quotation, “Light is therefore colour,” the polymer note is said to be stronger and more environmentally-friendly than the paper £20 it is replacing. It is, she says, the BoE’s most secure note yet.“The key challenge for the designers is getting something that both looks nice and can embed the various security features,” she says as she holds one of the new notes.

Turning it to and from the light, she shows how the words “Twenty” and “Pounds” change, then points to how a foil sky behind an image of Margate Lighthouse is blue on one side and silver on the other. All the polymer notes have raised dots that people with impaired vision can use to tell them apart.

“There’s no point in an anti-counterfeiting feature if no one ever checks it. What we want are really simple visual things that people can see just by picking up a note.”

The £20 note accounts for about 64 per cent of the value of UK notes in circulation and tends to be the forger’s note of choice. It is valuable enough to make the task worthwhile but not so large as to demand extra scrutiny from those being fleeced.

“We’ve seen very, very low volumes of counterfeit attempts at £5 and £10 polymer notes and what we have seen, have been quite easy to spot. We’re hoping that with the move to the polymer £20, we see another drop off in counterfeit levels. Polymer has proved to be much more difficult to counterfeit than paper.”

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The chances of getting a counterfeit yourself are “incredibly rare”, she adds. Fewer than one in 5,000 banknotes in circulation are fake and the BoE has been combating counterfeiters for almost as long as it has existed. “The first counterfeit notes came out a few days after the BoE was founded in 1694.”

Since then, there have been 33 chief cashiers taking on the forgers, three of whom have been women. For John, having been appointed chief cashier in 2018, the new note is the first to be issued in her name.

As chief cashier, she is responsible for the design of new notes; anti-counterfeiting; educating banks and retailers on identifying genuine notes; making sure there are enough banknotes in circulation; research and development into cash; and the BoE’s two cash centres, from where notes are distributed to banks, the Post Office and security firms.

There are 120 people working in her department on the BoE’s ground floor. Quiet but bright rooms are reached through dim passages with high, vaulted ceilings. John’s domain extends beneath the surface to the vaults, where behind a heavy steel door are billions of pounds of notes and gold. Cages are filled with tightly wrapped, plastic-covered packets of unused £50 and £20 notes, ready to replace worn notes or top up the circulation during high demand such as Christmas or a royal wedding.

Locked away are a number of £100m notes known as the “titan” and £1m notes called the “giant”, worth £4bn alone – they are kept out of circulation but represent official backing for the value of notes issued by Scottish and Northern Irish banks.

Note security is not the only challenge for the modern chief cashier. In 2017, cards overtook cash as the most popular form of payment. In 2018 (the most recent figures available) 28 per cent of payments were in cash, down from 87 per cent in 1985. Cash transactions are in decline and, in some areas, cash is less readily available but two million people still predominantly rely on cash for all their everyday transactions.

John is certain there will be a demand for cash, still the second most popular way to pay, for many years to come. “That can be because cash is good for budgeting. You get your money every week and once you’ve spent, it’s gone. A lot of people do still see cash as a contingency method of payment. For example, when we had the visa outages last year we saw people going to the cash point to get money out.”

“It’s highly unlikely,” says John. “Cash is still so important for a core sector of our economy.”

Looking forward, the BoE is going to “have to think about whether we should offer some sort of digital currency alongside physical currency,” she says. “The current crop of cryptocurrencies don’t really fulfil the properties of money very well. They’re not a particularly good store of value because their prices are volatile – and you don’t see that many shops accepting bitcoin. But this is an area where there’s a huge amount of innovation. We have to understand the challenges of a future when people are going to want to make those payments so we can position ourselves appropriately in that new world.”