THERE are lots of questions to ask today – questions about art and fakery and buying a Gainsborough for 120 quid – but the first question that occurs to me is: how do you end up in a place like this? We’re standing in a handsome three-storey gallery decorated with paintings with six-figure prices and above the front door, in big black letters, are the words “Philip Mould”. Mould himself jokes about feeling the weight of having his name in bricks and mortar on one of the grandest streets in London. The sign reads “Philip Mould”, he says, but what it really means is: there’s no turning back now mate.

In fact, Mould has being doing this kind of thing for a very long time now and seems pretty much relaxed about making decisions that can make him, or lose him, many thousands of pounds. The art dealer, expert on The Antiques Roadshow and famous exposure of fake art on the BBC series Fake or Fortune? is 55 years old and has been interested in antiques and paintings for at least 40 of those. He was only 12 when he started selling bits of silver at home in Liverpool and here he is selling portraits of Charles II and Henry VIII to hedge fund managers in the heart of the historical art world.

It turns out the answer to the question of how he ended up here is all to do with an eccentric woman he stumbled on in a shop 40 years ago. “My mother loved antiques,” he says, “but she was in a wheelchair – she got polio in her mid 20s. She was entirely disabled from the waist down but she managed to have two more kids (she’d already had two), and we became her sort of helpers. She would drive through town and she’d get her hair done every week and then we’d do the shopping. And she would send me into antique shops.”

It was on one of those antique shopping trips that Mould met the woman that would change his life. “She wasn’t born to be an antique dealer, she was a retired schoolteacher who grown-ups loathed and distrusted – she was bombastic and imperious, but kids were bewitched by her,” he says. “And she grabbed me as an 11 year old who wasn’t functioning at school and she changed my life. She got me to pick things up, she fired questions at me and jabbed me like a cattle prod for my responses on everything.”

Recalling the woman now, more than 40 years on, Mould credits her with kick-starting his artistic critical skills which he now uses on paintings to assess, attribute or reject. In his most recent book, Sleuth, which Mould will be talking about at the Boswell Book Festival at Dumfries House on May 8, he describes the moment he spotted a rather non-descript 18th century painting on eBay of a young gentleman. There was something about the way the paint was applied to the mouth and eyes that made Mould think of Gainsborough but the rest of the painting looked like it had been done by someone much less talented. So he bought the painting – for around £120 ¬ got it back to his gallery, and slowly applied acetone and white spirit to it and the over-paint dissolved like lard. What was revealed underneath was a genuine Gainsborough.

In the book, Mould’s description of the process of examining and revealing the Gainsborough brings to mind a forensic pathologist working on a body and there are some similarities. Increasingly, technology means that, instead of merely looking at pictures like we did in the past, we can look through them ¬ paint, says Mould, acts almost like blood at a crime scene.

Mould also believes new advances in technology mean that in the years to come we will have to re-assess paintings we thought we knew. In his book, he says the world is heaving with paintings that are unattributed or incorrectly labelled and I ask him how widespread he thinks the mistakes are. “I would have thought anything up to ten per cent are wrong or they are hopeful attributions,” he says. “You see, before art history became sophisticated, if something was 17th century and moody and Dutch, you’d just bang the name Rembrandt on.”

The downside of technology, he says, is that it will also be used by the fakers. “Perhaps the most chilling thing, and I didn’t think we were going to be going in this direction, is more I’m learning about fakes, the more I’m beginning to realise that some of the most sublime technological and creative developments in the world are now being applied to fakery.” I ask him if he has any grudging respect for fakers, in that it requires more skill than most crimes, but he’s not having any of it. “It’s just dressed up fraud. Yes, you’re taking in rich people so it’s a bit of a game you could argue, but theft and fraud is what it is. I think it’s grubby.”

Mould is perfectly happy to admit, though, that he is not immune from being caught out by forgeries and fakes, even though on his BBC show Fake or Fortune? he is seen confidently asserting the provenance of notable artworks. His office in his Pall Mall gallery – mahogany desk, thick black wallpaper, portrait of William Pitt on the wall – is the office of a successful, confident man, but he’s made plenty of wrong calls and cock-ups and he tells me about the most recent one last year.

“I went to a guy’s house to buy a collection of paintings,” he says, “and as I was going out, he said ‘would you be interested in buying a miniature?’ so I had a look at it and the rest of the paintings were rather great and I was happy to buy them and he was happy to sell them – I was on a bit of a roll because everything seemed to be going nicely so I said yes, took a quick look at it and it was in a rather nice old frame and I brought it back to the gallery and I couldn’t understand why my staff were smirking. I’d bought a photograph.”

Mould doesn’t beat himself about that kind of thing – in fact, he is rather interested in the psychology of the faker and the fake and cites the example of The Hitler Diaries, forgeries that were authenticated by the eminent historian Hugh Trevor-Roper. “His normal critical faculties went into spasm,” says Mould, “because this was something he wanted so much – he overlooked the fact that errors in history were repeated in the diaries, that the seals on the front of the diaries were plastic rather than Bakelite – cardinal mistakes because he wanted it so much. What keeps us going is that this rhapsodic pursuit and it can mean you live by the sword and you die by the sword – romanticism can catch you out.”

In person, Mould himself is a mix of romanticism and practicality, emotion and science, love and money who is very happy to tell you what he thinks. His London gallery is full of the paintings he loves, which pretty much means nothing after about 1930, so I ask him for his thoughts on contemporary art. Does he love any of it? Will any of it endure?

“I think that five per cent of what’s out there is stimulating and sustainable,” he says. “There’s far too much being produced that’s far too heavily marketed and there isn’t frankly the infrastructure. The artist has got to be alive to market these things, the art dealer needs to be committed, the writers and bloggers and critics need to be on board, you need to have a museum to enshrine some of these objects. Almost anything by anybody if it’s properly marketed in certain parts of New York is worth £25,000 – it’s got to an absurd degree. I’m not saying contemporary art is bunkum but what I am saying is that the majority of it is unsustainable.”

Mould knows he may be proved wrong on that, because mistakes and risk and mis-attribution is part of what he does here, but he seems to love the game. This gallery, with his name in bricks and mortar, is a big statement in a big city, but its owner seems to relish it – in fact, he thinks it makes him better. Nothing, he says, quite brings oxygen to the brain like financial risk.

Philip Mould is appearing at the Boswell Book Festival on Sunday May 8 at 2pm. For more information, visit www.boswellbookfestival.co.uk