Secrets of the Monet hunter

If you've got an Impressionist masterpiece tucked away, then this man would like to hear from you. In fact he's probably got your phone number

Paris traffic wardens, according to Andrew Strauss, tend not to issue parking tickets in the mornings. But they will in the afternoon. So it's in the first half of the day that Mr Strauss gets in his car, risks a few fines, and does his rounds of the city's owners of precious art. "I love seeing inside people's homes," Mr Strauss says. "I scan everything. I scan the objets. I scan the furniture. I scan the paintings. And of course I scan the people. You can learn a lot before you have even started to talk."

Mr Strauss is one of the art world's great persuaders. As the head of Sotheby's Impressionist department in Paris, it is his job to cultivate the privileged few in whose possession reside the treasures of the genre: the Monets, the Degas, the Sisleys, the Pissarros. In Mr Strauss's line of work, you are judged by the value of the art that you bring to market. And at the moment Mr Strauss is on a bit of a roll.

It was with an unmistakable note of triumph that Sotheby's last week announced that it would soon be auctioning one of the most spectacular achievements of the Impressionist era: Monet's 1906 Nymphéas, a painting in his celebrated waterlilies series that has not been seen in public since 1925. When it goes under the hammer in London on 24 June, it will have an estimate of £10m to £15m, putting it among the most valuable Monets ever sold. The Monet record of £18m – paid in 1998 for a painting of a Japanese bridge in the artist's garden at Giverny – might even be under threat.

When the cheque is handed over, it is Mr Strauss who will be justified in feeling the greatest satisfaction. For it was through him that Nymphéas finally came to light, completing a personal hat-trick which, from his headquarters on the rue du Faubourg Saint-Honoré, has also seen him secure for sale at Sotheby's the Degas pastel Danseuse au Repos (£17.6m in 1999) and Monet's Haystacks: Last Rays of the Sun (£10.1m in 2001). "France is a great hunting ground," Mr Strauss says. "There are thrills throughout the period of the chase. Well, I don't like to use the word chase. Throughout the period of becoming familiar with the picture."

Mr Strauss, 40, certainly has the background for the job, which he has held for 15 years. His great-grandfather, Jules Strauss, was a banker in Paris at the end of the 19th century who built up a collection of great Impressionist paintings. The collection was sold before Mr Strauss's father Michel, Jules's grandson, established himself as Sotheby's leading expert on Impressionism. Andrew, born and brought up in London, learnt at his feet, joining Sotheby's after abandoning university two years into his degree.

"I remember as a child being dragged round the great galleries of Europe," he says. "My father usually planned our holidays so that we went to Paris or Amsterdam or wherever. But something must have sunk in. I found that when I joined Sotheby's I would come across works and realise that I'd seen them years before."

Now, however, there is a twist to the relationship. After 39 years of helping to turn Sotheby's into a global concern, Michel Strauss left it in 2000, only to turn up this month as the head of paintings at Phillip's, the auction house with designs on challenging Sotheby's and Christie's. Poised and self-possessed, Mr Strauss lets slip a sigh as he acknowledges that he and his father are now rivals. "But business is business, and home life is completely different," he says. "We know we are not going to slag each other off."

Mr Strauss describes as "a blip in the company's history" the controversy over price-fixing with Christie's, which resurfaced last week when the EU announced it was launching an inquiry into the auction houses' alleged arrangements. Meanwhile it has fallen to him to maintain the trust of those who might like to consider selling their pictures through Sotheby's.

Mr Strauss is particularly proud of the tactical coup behind the Monet Haystacks sold by Sotheby's last year. As much as 10 years previously, his detective work had identified the Parisian who was the likely owner. So he wrote to him, enclosing a couple of Sotheby's catalogues. The weeks went by without a response. Mr Strauss's understanding of French etiquette told him that a follow-up phone call would have been counter-productive. Instead he continued to send the man catalogues, still unsure whether he really did own the Monet. The weeks turned into months, which turned into years. Then, one day, Mr Strauss's target broke cover. He did own the Monet. Would Mr Strauss be interested in selling it?

The story behind the latest Monet is different. The most recent record of Nymphéas showed that it had been bought in 1940 by one Victor Hélin, of whom Mr Strauss knew nothing other than that he had made a lot of money in the 1930s "patenting something". By now the painting would have passed down through the generations, its ownership and location unknown. So when a man representing Mr Hélin's descendants contacted Mr Strauss to talk about a sale, he could barely contain his excitement. "I almost didn't dare believe it was true." Other works on offer made it read like "a dream shopping list".

Unusually for paintings of this quality, they were not in Paris, but "in a small town in the middle of France", safe in a warehouse. The Monet was in perfect condition, unvarnished, and without the second canvas backing that so many paintings of the time were given to help to preserve them but which also had the detrimental effect of flattening the surface. "It looked as if it had just come out of Monet's studio," Mr Strauss says. "It was just a beautiful painting." And it was Mr Strauss's to sell.