Sandro Grainger is probably the most beautiful human being I've ever met. Or ever expect to. Sitting opposite him in the upmarket Kensington restaurant he'd chosen for this meeting, I felt as though I was lunching with a flesh-and-blood Botticelli angel. Plentiful blond-tipped brown hair pushed carelessly back from his face, skin the colour and consistency of beige satin, chocolate-brown eyes surrounded by thick gold eyelashes, a full Pre-Raphaelite mouth. Magnificent. And not in the least bit effeminate. A perfect blend of his parents: Dominic's classic Anglo-Saxon good looks mingled with Maddalena's dark Mediterranean beauty.

I'd met them many times, since Maddalena was a cousin of my Italian brother-in-law, Carlo. I'd even spent holiday time at their villa on Corfu, along with Carlo and his wife, my sister, Meghan. The Graingers were legendarily rich. As well as Corfu, there was the house in Rome, the house in London and an apartment in New York. Plus vast tracts of southern England.

As for Sandro, I'd known him for years, first as an adorable toddler, then as a beguiling and – mercifully – pimple-free adolescent, and then as an almost-adult in his early twenties. We went back a long way, but sadly only on an occasional and casual basis. I wasn't quite old enough to be his mother, unless I'd been knocked up by some perv. So it came as something of a surprise when he'd telephoned and asked me out for lunch. Flattered? Not really. Obviously he wanted something.

I swallowed the last morsel of my chocolate cheesecake, wiped my mouth with my starched linen napkin and leaned back in my chair. 'OK, Sandro,' I said. 'Nitty-gritty time. So spill.'

His eyes slid away from mine. 'Uh ...' he said.

'Your company is delightful,' I said. 'And I've very much enjoyed our extremely good lunch. But I can't kid myself that a twenty-five-year-old guy such as yourself would seek out a woman of my age if he didn't have some kind of an agenda. Am I right?'

He squirmed. 'Uh ...'

'So level with me.'

He fiddled with his water glass, then with the salt cellar in front of him. Picked up his dessert fork and put it down again. 'Thing is, Alex ...' He fell silent.

'Yes?' I encouraged.

'Look, I know ...' Another silence. He rootled in a pocket, brought out a small Swiss Army knife, put it back. I got the impression that he was not at ease.

'Which is more than I do,' I said.

'Uh ...' He cleared his throat. 'You used to be a police officer.'

'This is true.'

'So you must be something of a detective.'

I smiled. 'And you want me to do some detecting on your behalf?'

His face relaxed. He gave a half-laugh. 'Exactly.' He gazed round the room like someone whose troubles were finally over.

'This is fascinating stuff, Sandro. But could you give me some further details?'

He thought about it, then sighed. 'Yeah. I guess I have to.'

'Otherwise there's not a whole lot I can do.'

'I quite see that.' More nervous fiddling with salt and pepper. More twisting of the water glass.

'Sandro!' I placed my hand on top of his. 'For goodness' sake, tell me what the problem is.'

'OK.' He nodded fiercely to himself. 'So, earlier this year, I was staying in my Uncle Cesare's place in Venice, while he and my aunt were attending some high-level meetings in Geneva. He is the Marchese Cesare Antonio de Farnese de Peron, to give him his full title, and has this rather grand apartment in one of the palazzi along the Grand Canal. And I decided to hold a dinner party, you know, a real grown-up dinner party, to celebrate my birthday. Catered, private chef, black tie and all. My generation tends to go round in ripped jeans all the time, so I thought it would be fun to dress up a bit.' His eyes were wide with earnest sincerity, while I thought that many of his generation probably weren't able to afford much more than jeans, ripped or otherwise.

'Sounds great. How many people did you invite?'

'Just ten. Well, nine really, since I was one of them.' He laughed nervously. 'Was going to be eight, but at the last moment Tony – my girlfriend's brother – decided to come after all.'

'And all went well? Nobody threw pasta con vongole at the priceless tapestries, or drew a moustache on the Contessa's portrait?'

'Of course not!' He seemed shocked. 'My friends are all very ... beneducato. Well-bred. They know how to behave.'

'Even well-brought-up young people can go off the rails.'

'Of course. Anyway, the next day I had cleaners in to make sure the apartment was immaculate before my aunt and uncle came back. Naturally I'd asked my uncle's permission to hold the party, because if there's one thing I don't want to do, it's to get into Cesare's bad books. He can be ... fierce.' Sandro shuddered dramatically. 'I mean fierce!'

'So what do you need me for?'

'Well ...' Sandro hesitated. 'I still can't quite believe this, but I was walking down New Bond Street the other day and I noticed a ring in the window of a pawnshop. It looked exactly like a ring that has been in my uncle's family for generations – an heirloom supposed to have been given to one of my aunt's ancestors by some doge or other way back when, as a mark of his esteem or something. And then, when I looked closer, I could see that in fact it didn't just look like my uncle's ring, it was my uncle's ring – or my aunt's, to be more precise. Right there, in a pawn shop!'

His privileged young voice displayed scorn and disbelief at the very notion. Did he have any idea of the hand-to-mouth existence led by some sectors of society? How much reliance some people placed on pawnbrokers? Or, for that matter, what a respectable history the pawnshop possessed? Didn't sound like it.

I cleared my throat. 'Sandro, let me give you a little background here. Pawnshops are not all about furtive criminals slinking in to try and fence stolen property. Nor are they men pawning the family teapot in order to get drunk before staggering home to some sordid slum in order to beat up their wife and kids. In fact, in the United States, they're often referred to as deluxe collateral lenders. They're patronized by all sections of society, people looking for short-term loans, from high-end rollers and doctors, to lawyers and even bankers, believe it or not. And the collateral these punters offer can be as upmarket as wine collections, or fine art, or cars. Even uncut diamonds. And for your further information, Isabella of Spain used a pawnshop to finance Christopher Columbus's expedition, and whichever French king it was pawned the royal jewels to raise money for the war against Henry the Fifth. There's even a famous pawnshop which was in fact a charity.'

'How come you know so much about it?'

'I had to do a research project when I was at uni,' I said.

'Well, what about this ...' He choked slightly with indignation. 'A couple of days after I saw the ring, I was in an art gallery, looking for a gift for my mother, and there on the wall was a small Botticelli, a young man, head and shoulders, with a pastoral scene visible through the window behind him. Very delicate, very lovely.' He coughed. 'I couldn't believe my eyes. I went in – and again, it was my uncle's!'

'You're sure?'

'Well, to be honest, it was probably painted by an apprentice, rather than the master himself, but you'd have to be an expert to spot it....