The man who has probably saved British consumers more money than any other in history has agreed to play a little game with me. Which of us has spent less on what we are wearing?

“I have problem feet so I buy Skechers,” Martin Lewis begins, pointing at his square-toed black lace-ups. “I bought these buy-one-get-the-second-half-price so the effective price was about £35. The jeans were bought in Zara in Spain in a sale and were about £15. The shirt . . . could you tell me where it’s from and I’ll tell you how much it is?”

He leans forward, I put on my reading glasses and peer down the back of the Money Saving Expert’s clean neck to see the label. Saccor, I read.

“They do a special deal that if you buy one you get one free — it’s a £60 shirt so it’s a £30 shirt. This underwear came from Amazon so they’re probably about £2 and the socks were a gift from a friend.”

In return, I tell him I’m dressed entirely by eBay: shoes, £50; jacket, £10; shirt, 99p; trousers, £8. Outlay £68.99. Value of clothes if new: about £850.

“Lovely!” he says. “Great!” The fact that I have been thriftier than Mr Money Saving Expert doesn’t appear to trouble him at all. His aim in life, he explains, is to teach people to keep money in their pockets. If I can do that unaided, he’s happy.

This mission has made Martin Lewis the most successful journalist in the world, ever. He would doubtless disapprove of such a claim, as his journalistic style is never to say anything unless it is backed by hard data. My style is rather looser, and although I have no evidence that the above is true, I’ve rummaged around for examples of people who have been more successful and found none.

In 2003, when he was just 30, Lewis set up a website with the slightly plodding name Money Saving Expert. His outlay was £100. Twelve years later, having sold the site to MoneySuperMarket for £87m in 2012, he is worth more than £100m. Not even Richard Desmond has matched that in terms of growth and while the latter has got rich through exposing celebrity cellulite, Lewis has done it by giving helpful advice on how to cut your electricity bills — and become a celebrity on the strength of it. In 2013 he was in the top 10 most searched for people in the world, just behind Taylor Swift, but ahead of Beyoncé.

At first sight, Lewis seems outrageously normal, with the bland looks that work well on telly. He is a middle-class boy from Manchester, son of the headmaster of a special needs school, who was good at maths, went to the London School of Economics and from there — via a stint at a financial PR company during which he worked out he was “on the wrong side” — to study journalism at Cardiff University.

Yet, after about 30 seconds in his company, it is clear that he isn’t normal at all. He sits on the edge of an overstuffed chair in the lobby of a London hotel, talking at twice the usual speed and volume, every sentence filled with facts and figures. He’s trying to tell me how many people get his weekly email and checks his phone for an exact number — 10,781,000 — only to explain at once the weaknesses in the figure and offer 8,750,000 instead.

A colleague of mine who was in the same year as Lewis at journalism school describes how his loudness and nerdiness were then viewed with condescension. Six months after graduation my colleague (who had landed a good job on The Sunday Times) recalls bumping into Lewis at a party and listening pityingly as he said he was doing personal finance for some cable TV station, but that the great thing was he got to dress as a pirate to jazz up the story.

I’ve got nothing left to prove to anybody. I’ve made a lot of money, I’ve done my charity stuff, I’ve got my awards. I’ve done everything I could’ve ever wanted

Lewis now employs 40 journalists — some of whom are former classmates. He has done so well that in March he will give up day-to-day journalism, becoming executive chairman of the site, to spend his time on charity work and consumer activism instead. “I’ve got nothing left to prove to anybody. I’ve got the prime-time TV show, I’ve had the best-selling book, I can have a column wherever I want to write it, my website is the biggest thing since sliced bread, I’ve made a lot of money, I’ve done my charity stuff, I’ve got my awards. I’ve done everything I could’ve ever wanted. I now have the beautiful freedom that I’m wealthy enough that I decide why I get out of bed in the morning.”

So why does he? “It’s somewhere between ego and because I believe it’s morally right and it’s what I was born to do.”

I suspect he is propelled out of bed every morning because he is physically incapable of doing anything else. His publicist has ordered him a coffee and a Diet Coke but he says his doctor has banned him from drinking either. He has been suffering from stress, and his stomach is not able to cope with anything stronger than a mint tea.

“I grew up in a school where people at 15 learnt to tie their shoelaces. You cried because they’d spent five years learning to do so — and that’s where you learn how fortunate you are. There’s a lot of that that sticks with me.”

So he set up Money Saving Expert, an Aladdin’s cave for the thrifty, which teaches consumers everything bar how to tie their laces. It tells them which credit card to have and which mortgage. There is stuff on how to budget, how to complain, how to get a refund on your council tax and how to sue the council for the damage potholes have done to your car. There is an introduction to the world of extreme couponing (“I got £67 worth of groceries for 11p”); there are special offers (12 bottles of nail varnish for the price of three) and there is even advice on whether it is OK to use a two-for-one voucher on a first date. It is, says the website — basing its answer on a survey of 8,000 people, 85 per cent of whom confirmed that such penny-pinching was not romantically offputting.

The site is chatty, commodious, helpful and very clear. Yet the scale of its success — with 15 million users — needs some explaining. Lewis warms to the task of doing just that. For a start, he was the first person to take money journalism away from unit trusts to include everything people spent cash on — from credit cards to alternative tampons. Second, he never quotes anyone. All the research is done by him and his team of journalists. Third, he realised that the only thing people want is answers. “Nobody wants to know about the issues. They want to be told what to do, and I tell them. That’s the difference.”

Martin Lewis receives his OBE on February 13 at Buckingham Palace, alongside his wife Lara Lewington

A fourth thing was having his face all over everything. “Finance is a personal thing and you want to see the whites of someone’s eyes. Thankfully, our research shows there are a lot more people who like than dislike me — it’s about an 85/15.”

His research also shows that when his face comes off the site in March, people will still go on visiting, and the site will go on raking in the money. The business model is a stroke of genius: with no advertising and no subscriptions, all revenue comes from paid-for links. What Lewis has pulled off is to convince everyone of his independence. He recommends only the best deals — whether the company happens to be paying him for a link is irrelevant.

It is almost too good to be true. He makes money and consumers save — in the past 12 years he estimates that he’s saved them “tens of billions” of pounds. “You can argue there’s over 20 billion been paid out in PPI and we reckon we’re over 25 per cent of that, so there’s five billion to start with.” Indeed, that very day Lewis has been with the Financial Conduct Authority fighting to the last to get compensation for consumers who were missold Payment Protection Insurance.

All that campaigning is splendid; only doesn’t the site also serve to encourage overconsumption? No one needs 12 bottles of nail varnish. Quick as a flash, he replies: “That’s why I have Martin’s Money Mantras: Do I need it? Can I afford it? Will I use it? If not, don’t buy it.”

In any case, the purpose of the nail varnish deals has nothing to do with nails. It is to hook people in and lead them towards bigger savings on utilities and other, duller things. “My job is to tweak your nipples until you do the important stuff. I have an absolute rule; we are not here to give the user what they want, we are here to give users what they need and often people don’t know what they need.” As he says this, he looks so intense that I’m not at all surprised that he’s got an incipient stomach ulcer.

15m Number of website users

8,750,000 Subscribers to weekly email (estimated)

£87m Amount sold for in 2012

I tell him about my own approach to money. I have a very rough sense of how much I can spend without being overdrawn — and that’s it. The reason I buy everything on eBay is because of the thrill of the chase. There is no thrill to be had with electricity bills, so I don’t bother.

Lewis gives an irritable wriggle in his seat. “You are the hardest sort of person to help. The average time it takes to switch gas and electricity — we’ve done a test on this — is four minutes and 12 seconds. A typical saving is £290. Now, the FT pays well but it does not pay you £18,000 an hour.”

I say I don’t look at it like that. “Well, you’re wrong. You’re irrational, I’m not. Would you give up four minutes and 12 seconds of your time to raise £290 for the refugee crisis in Syria?”

I would, I say. “So go and do a bloody gas and electricity comparison and give them your savings.” He is practically shouting at me.

I persist that the ultimate point of having money is that you don’t have to think about it. Which means there is a right amount of money to have: too little, you need to think about it all the time. Too much, ditto. Lewis almost jumps out of his chair in protest. “Not thinking about money is the greatest sin in Great Britain! It leads us to be poor consumers and leads us to overpay. We should all go around asking each other what things in our house cost. Oooh, you’ve just got a new telly — how much was it?”

I say I would hate it if people kept asking how much things cost. It is embarrassing. Nonsense, he says. “It’s empowering.”

In that spirit, I ask how much he’s worth. “Nine figures is what I publicly say, I’m comfortably in nine figures.”

I point out that this is not a very helpful estimate. It covers everything from 100 million to a billion. “I’m saying just over 100 million. The reason I’m not giving you an accurate number is because 40 per cent of it is in movable assets that I can’t . . . If you ask me in 10 minutes’ time, it’ll change.”

But isn’t it stressful having to worry about such substantial sums? “It’s joy, it’s joy that I can do what I want.” Having money hasn’t changed his attitude at all. He loves value as much as ever, and still sometimes goes to pound shops.

He is never irrational about money, he says, except when he sees something his three-year-old daughter would like. He then mentions something else he bought recently that wasn’t great value, but asks me not to print it. It’s private, and it wouldn’t look good reprinted in The Sun.

“I just don’t want to let people in. The Money Saving Expert does interviews; Martin Lewis doesn’t. I want some stuff that’s for me and for my wife. We met — we’re both on telly [she is the TV weather girl Lara Lewington] and nothing makes me shiver more than being called a TV couple. We’re just two people who happened to fall in love and to work in the same industry. I don’t ever want us — it’s a great way to get divorced — to be a TV couple.”

I ask if he thinks that the rich (many of whom use his site) are even more screwed up about money than the rest of us. He thinks it depends. “A lot of City people are crap with their own cash. Because they don’t focus on it and they think earning is more important.” But what he hates is meanness. “I have a friend who’s very, very wealthy and doesn’t ever give to charity; I find that really difficult because I say, what the hell do you need that much for when you know what you can do with it?”

Lewis tells me he has just spent £4m setting up a mental health and debt policy foundation — the mentally ill are about five times more likely to be in financial difficulty than the rest of us. “People can spend £15,000, can spend an annual income on shit in three weeks in a bipolar spending spree.” What his charity is trying to do is find ways of stopping the spending before too much damage is done. He explains how it might work, speaking even faster in his enthusiasm. “I can make change happen,” he says, and I believe him. I think he can.

Whether he can change me is another matter. Before he goes, he asks me to make a solemn promise that I’ll sort out my energy bills. I say I will and when I get home I type in the details into his website (admiring the crafty way that the model works so that the website gets money, and so do I), and quickly find that I could save £360 a year by switching. I’m impressed. I plan to do something about it. Just not now. I have to write this article first.

lucy.kellaway@ft.com. See Claer Barrett’s Serious Money column in FT Money

Photographs: Carl Bigmore; Getty Images