"Strangers are just friends we haven't met yet": a treacly Americanism that no longer holds sway in our city centre, where strangers, too, are separated into classes, and the chances are that a modern Samaritan would be more likely to cross the road to help if the victim were dressed in Armani rather than Asda. On the streets and trains lurk all those people we don't want to know - ever. In restaurants and bars linger the sorts of men and women we quietly pray will introduce themselves to us so that we can (by some sort of social osmosis) enter their social group.

At a TV awards ceremony, a young singer amused guests by noisily sniffing the air around the champagne-swilling moguls and breathing out with a blissful sigh. "Can you smell it?" she asked two nervous BBC producers. "Eau de l'argent, baby, eau de l'argent." Standing nearby, I knew exactly what she meant. The air around them was autumn fresh, set off by the tangerine undertone of several fruity egos. Desperation has a far less rare and palatable perfume.

Sitting in a Greek cafe in Finsbury Park this week, I felt a pair of strange, alien eyes boring into my back and felt the familiar sense of inevitability. After a while, there was a raspy, throaty: "Hello. Hello?"

"'Scuse me?" I began the routine designed to deter the dangerous, the desperate and the downright boring: to coin a colloquialism, I "copped a deaf 'un". Opposite me, my husband peered past my shoulder and gave a tiny shake of his head. "Don't turn around, for God's sake," it said.

The plaintive bleating continued: "Please, please, 'scuse me dear . . ." The owner of the cafe turned the music up to drown her out, which didn't work. I swivelled around and, in my best "I'm being polite but make it quick" tone, said: "Ye-es?" There was Janice, bearded, 60-plus and wearing a child's dirty, fur-hooded jacket. Her hair hung down in damp, aged coils, and her greasy Alice band matched her coat.

"Look!" She rifled through a plastic bag. I stifled the urge to turn away and ignore whatever futile bauble or relic from her awful life she wanted me to see. She pulled out a brightly beaded purse. Proudly, she thrust it in the air. "Lovely," she smiled, revealing yellow teeth.

I gave her a stock-in-trade, patronising "Yes, mmm, lovely", and turned back to my giggling partner and cold eggs. Suddenly, something struck me on the head. I pulled a paper aeroplane, clumsily made out of a serviette, from my hair. By now, Janice was heaving with laughter. She had me.

She knew that she had managed to engage me. Now I was interacting with her, listening to her ravings. How many days, weeks, had it been since someone had actually stopped and chatted to her before brushing her aside? In winter, it's as if the city becomes full of the lonely elderly. They cluster in the Post Office and harangue newsagents about the weather and the bus service.

Janice really needed to be listened to. I stifled the overwhelming urge to tut, and made origami shapes and planes instead. We "played" for five, maybe ten minutes. It seemed like an eternity. She modelled a pretty good seagull with one napkin - it was headless, granted, but at least it was recognisable. Finally, she got bored with me. I made as if to sit back down, when suddenly my husband gasped at something over my shoulder. "No, no!" he screeched.

I span around, and saw something glinting in her hands. "Shit!" My arms flew up to cover my face.

But what I'd taken to be a knife was only a pen, and Janice was sitting peacefully at her table, writing. How we laughed. We chuckled, too, about her scruffy beard. "She should at least trim it," joked Craig.

Then, as we were leaving, Janice thrust an envelope into my hand. "Open it, open it." The Christmas card said, in sprawling child's letters: "Mery Xmas. To my freinds. Love Janice."