Van Dyck or Van Dupe? He was thrown out of the Society of London Art Dealers. But it was for a time extremely unpleasant. He's a second-generation main dealer in English portraits and we sort of overtook him. I imagine he was really ------ off. [My] being high profile as well, doing the books, the TV... you don't necessarily make friends doing that. Mould counters accusations that he is a Fifth Columnist by saying that shining a light on the workings of the art world is actually doing the art world a favour. In the first programme of the first series a once-questioned Monet was proved, beyond what most experts thought was reasonable doubt, to have been genuine. Yet the Wildenstein Institute in France, generally considered to be the presiding authority on Monet attributions, insisted it was a fake. Mould says that one series of Fake or Fortune? on, the penny appears to have dropped and the programme's process of attribution is now taken seriously by the academic authorities - as certain events in the second series prove. "We've had some dramatic successes," he says. What has happened is that the programme has developed a power of its own. It is a transparent and more democratic way of executing a process which up until now has been rather opaque. That process includes detailed forensic analysis - "the CSI bit," as Mould calls it. Dendrochronology (dating of the wooden panels beneath the paint by tree-ring analysis), as well as spectrometry (analysing the paint itself to establish age) are both central to the attribution process. The results can be wondrous, Mould says, but they will never be infallible. You can home in far closer on a date, but can you 100 per cent conclusively say something's right or wrong? Because a faker could do all the research you've done and give the impression of age and time. Fakery, he says, is on the increase. The internet is both a help and a hindrance - there are more discoveries than ever thanks to the web, but detailed digital imagery makes forgery easier, and internet auction sites provide a simple, semi-anonymous outlet. It's almost impossible to recreate the strokes of a genius technician of the 17th century. But fakery in 20th century paintings, particularly British paintings, I see all the time. Think how easy it is to do Lowry. I could do you a Lowry now. I've seen fake Francis Bacons with old labels on the back. I see them all over the place. Mould, who says that he "loathes" forgers ("I think they're just shabby crooks") says he knows who the master fakers are in his own area of British portraiture. One day I want to take them on. In a future series I'd love to expose one or two of them. The problem is he has to be absolutely sure he's right. There's nothing more embarrassing [than] to set out on a task only to ------ up by the end. Mould is renowned for the ones he's got right - the only extant portrait of Henry VIII's elder brother Prince Arthur, which paid for Mould's Kensington town house; a recent coup that he says could "take care of my old age" - currently waiting for official attribution; the earliest formal portrait of a transvestite which he discovered last year and sold to the National Portrait Gallery; and the Van Dyck of a lost child that now hangs at the Ashmolean Museum, Oxford. We buy three paintings a week from screen [over the internet]. It's a bit like tasting food with a cold. It's part of the risk. I do make mistakes. Which brings us back to the Van Dyck. Fake or fortune, Philip? He laughs the say-nothing laugh of a man who has become very good at keeping secrets, and offers me another cup of tea. Fake or Fortune? returns to BBC One on September 16 at 6.30pm