From armed hold-ups to the Antiques Roadshow

In a way, the show's experts are almost like doctors. They are specialists in diagnoses as well as in spotting diseases - in their case, the flaws that could make all the difference between fortunes and failures. They have to be able to recognise immediately a piece's history as well as its value. "It's unlikely I'd be stumped," says Benjamin. "You have to realise we'd only choose something about which we have some knowledge. It's important to be accurate. We can't quote prices that are too high or too low."

The experts have to be personable. Yet if John Benjamin is anything to go by, they don't have to have been brilliant at school. "I was always at the bottom of the class," he admits. It was his mother, Doris, who suggested he go into something connected with the fine arts.

She saw that he did not want to join his father Berry Benjamin in the family knitwear business, and encouraged his appreciation of art by going to galleries and museums with him.

Also, in a typically Jewish way, even at 55, his parents are still his most loyal fans - apart, of course, from his wife Tricia and their 18-year-old twin daughters, Georgia and Olivia.

He was brought up with his elder sister in Wembley, north-west London, went to school in Harrow (not exactly at Harrow, but at the nearby John Lyon School) and attended Middlesex New Synagogue (which he still does on Rosh Hashanah and Yom Kippur).

It was Doris's advice that led him to an apprenticeship at jewellery shop Cameo Corner in Bloomsbury, which was run by the Polish exile, Moshe Oved - "an extraordinary man", says Benjamin. Before long he was able detect a when a piece was craft, and who made it. He also acquired a knowledge of the dangers of the business - and not just the risks of making a mistake with a valuation. Once he was held up by a gang of robbers who lined the entire staff of the shop against a wall. Then, a few weeks after moving on to work at Phillips, the auctioneers, he had a shotgun pressed against his ear by a member of another gang of raiders and told "I'm going to blow your head off."

"It affected me badly at the time and still affects me," Benjamin says.

But there were good things that came out of it, too. Fifteen years ago, he was asked to go to Sevenoaks to value a collection of jewellery belonging to a local woman. She had been keeping it in a shoebox under her bed. It turned out to be a treasure trove made by her jeweller father, Henry George Murphy, who died in 1939. "I was stunned by the virtuosity of design, enamel, gold, silver, or design technique, complexity of form," he recalls.

Benjamin photographed the jewels. Eighteen months on, there was a phone call. The shoebox and its contents had been stolen - uninsured and untraceable. A year later, "on a Friday afternoon, a woman came into our place in New Bond Street, wearing a beige towelling jump suit with mink trim. She had a Coronation biscuit tin. I opened it and there, swimming beneath, was all the jewellery that had been stolen. The extraordinary thing was that day I had the slides of that jewellery on my desk. I was the only person in the world not to bring it to." The woman was, of course, arrested and the jewels returned.

His was a meteoric rise at Phillips, becoming head of its international division while still in his 30s. Later, he was made redundant. He now sees that as the best thing that could have happened to him. He established John C Benjamin, Ltd, a name under which he travels the country making valuations for people who need to know either the price at which they could offer their treasures for sale or to list in their insurance policies. "I do my share of trips to St John's Wood," he says, without being more specific.

He thinks that the long-standing Jewish connection with the jewellery trade is something of which to be proud. "It has been very important in the history of Jews, especially in Britain, when you go back to the days of Black Lion Yard [once a centre of jewellery activity in London's East End]. The bedrock of the jewellery business, selling diamonds, for instance, has always been Jewish. It's part of the plasma in the blood and should be triumphed."

The work valuing jewellery led to another essential part of his life. He started lecturing, which he says he loves. "I learnt very early on the need to entertain people - something I got from my father, who has a wonderful sense of humour."

Occasionally he and other members of the Roadshow team talk on cruise ships. "What is the thing that people bring on cruises? They don't bring furniture. They bring jewellery - and there's no escape, short of jumping off the boat, from these women who come up to you at 12 o'clock at night and say: 'Could you value my diamond ring?'"

The next edition of ‘Antiques Roadshow’ is on BBC1 on Sunday at 8pm